Low KH/pH cause of stalled cycle?

Hi all

I'm doing a fishless cycle in a heavily planted 60l. tank with an inert substrate.
My tap water readings are NH3 0/ NO2 0 /NO3 0/ pH 7/ KH <1/ GH 4

Because of the low KH and the pH dropping in the tank I've been adding potassium bicarbonate every 2-3 days. 5ml bicarb takes the pH from 6 to 7.5, and the KH from <1 to 4.

The cycle seemed to be ticking over nicely, with added ammonia being processed quickly down to 0 and NO2 down to 2. For some reason I didn't add potassium bicarb for 5 days. NH3 became stuck at 1 for a couple of days, where previously it had processed quickly. Assuming the cycle had stalled I did a 20% water change (the first one I've done during the 25 days of the cycle, to replenish minerals) and added more bicarb (I realise it's not a good idea to change two parameters at once, but I was anxious it would stall completely).

Readings from the last 4 days -
NH3 1/ NO2 0.5/ NO3 20/ pH 6
NH3 1/ NO2 0.25/ (did water change, added bicarb)
NH3 0.5/ NO2 1.5/ pH 7/ KH 2
NH3 0/ NO2 0.25/ NO3 20/ pH 6.5/ KH 2 (added ammonia and bicarb)

I have three questions -

1) I suppose the answer is probably 'either', but is the low pH or the lack of fresh minerals from new tapwater likely to have caused the stall?

2) Will the pH/KH problem continue beyond the fishless cycle? As I sometimes won't be around, it's unrealistic that I could continue to maintain a higher KH by adding bicarb every couple of days, and I'm concerned anyway that the pH swings it creates might harm fish.

3)I'm aware of people using bags of coral sand/Aragonite in the filter as a longer-lasting solution. Does anyone have any idea how much I should expect to use in a 60l. tank to raise KH to a manageable level, and how long I should expect it to be effective?

Any advice greatly appreciated.

 

Hey!

I had issues with low pH/KH causing a stalled cycle when I cycled my first tank.
I added some kind of buffer to raise the pH to 7.2, and that helped. I think it's the pH that stalls the cycle. Low pH causes ammonia to become ammonium, which bacteria can't process as easily. They struggle below 6.5, and when it gets too far below 6, they almost completely stop.

After I finished cycling, I stopped adding buffer. And it's been stable since then, as nitrates don't build up much (it seems to take more nitrates than nitrites to cause a crash ime) and I stay on top of water changes. My pH now sits at 6.6 when I test it each week. I could be wrong though, as I am a beginner, and both my tanks are reasonably heavily planted and I often see nitrate readings of 0ppm.

I've not tried using aragonite/limestone/coral sand in the filter, so I can't offer any experience on that.

 

Hey!

I had issues with low pH/KH causing a stalled cycle when I cycled my first tank.
I added some kind of buffer to raise the pH to 7.2, and that helped. I think it's the pH that stalls the cycle. Low pH causes ammonia to become ammonium, which bacteria can't process as easily. They struggle below 6.5, and when it gets too far below 6, they almost completely stop.

After I finished cycling, I stopped adding buffer. And it's been stable since then, as nitrates don't build up much (it seems to take more nitrates than nitrites to cause a crash ime) and I stay on top of water changes. My pH now sits at 6.6 when I test it each week. I could be wrong though, as I am a beginner, and both my tanks are reasonably heavily planted and I often see nitrate readings of 0ppm.

I've not tried using aragonite/limestone/coral sand in the filter, so I can't offer any experience on that.

Thanks for the response. Do you know what your tapwater pH/KH are? I'm just concerned that as my tap KH is so low I'll always have to use some sort of buffer.

 

Thanks for the response. Do you know what your tapwater pH/KH are? I'm just concerned that as my tap KH is so low I'll always have to use some sort of buffer.

My tap pH is 6.6 as well. I don't know the exact values of the KH. I just suspect it's low bc it doesn't seem to take that much to alter it. Seeing as since cycling, I've not had many issues with stability, I've not seen a need to know it precisely. Maybe it would be interesting to find out though.

In my second tank, I found I had success with just changing the water whenever the pH dropped and then re-dosing the ammonia. I don't know if my water might have slightly higher KH since I moved, but seeing as I only moved half an hours walk away within the same city, I doubt it. Before I moved I vaguely rememeber using a test strip and KH coming up near enough blank, but I could be misremembering.

All this chatting makes me really want to get more strips just to get an up-to-date idea. It sure would be interesting to know.
I remember in my old place, once I hit the nitrite stage, my pH pretty much just plummeted straight down to below where the test could measure as soon as I got above... 2ppm? But I can't speak too much for my current place as I added buffers before it got beyond 2ppm, and in the other tank I changed the water whenever it got that high. Plus I didn't dose ammonia as high in the second tank. And in addition I added some media from the other tank to help the process.
And in my old place, I never actually added fish. I gave up on the aquarium after several months of mistakes with cycling and didn't pick it up again until a few months ago.

Sorry that the reply was long and rambly. I'm very tired today, and rather stretching my knowledge of these things.

 

You have different bacteria that live at a PH of 7 than a PH of 6. You can cycle your tank at either PH but not both.
Constantly chasing these water parameters is not a game that you want to play.
At some point you will misdose overdose or forget to dose and everything comes crashing down.

Ammonia is basic, it could raise your PH when you add it.

My best advice would be to find a way to get your PH to stabilize without using solutions or powders or products that instantly raise or lower it. Crushed coral might be the solution, the length of its effectiveness depends on the acidity of your water. So test test test because it will expire. Therefore, i would keep it in my filter and not use it as a substrate if you choose to go that route.

 

You have different bacteria that live at a PH of 7 than a PH of 6. You can cycle your tank at either PH but not both.
Constantly chasing these water parameters is not a game that you want to play.
At some point you will misdose overdose or forget to dose and everything comes crashing down.

Ammonia is basic, it could raise your PH when you add it.

My best advice would be to find a way to get your PH to stabilize without using solutions or powders or products that instantly raise or lower it. Crushed coral might be the solution, the length of its effectiveness depends on the acidity of your water. So test test test because it will expire. Therefore, i would keep it in my filter and not use it as a substrate if you choose to go that route.

Thanks for that. Crushed coral seems the way to go. I previously had a disaster when I was advised by the aquarium shop guy to add what was sold to me as 'coral sand' to the substrate. It didn't seem to affect the KH but sent the GH sky high, and I had to scrap the whole thing and start again. So from now on anything I use will be staying in a bag in the filter.

 

^^^ Yeah, a lot of what was said above. With just 1dKH (17.9ppm), your bacteria will be able to process just a hair more than 2ppm of ammonia ( 1ppm ammonia takes 7ppm of KH to process), with less than 1dKH....they aren't going to be able to do much.

 

Welcome to Fishlore

Water changes alone may be all you need. Since it had been 25 days since you set the tank up the cycling process had probably depleted the original minerals present in you tap water. Change out at least 75% of the water and the water you are replacing should get the minerals back up and the pH stabilized to the level of your tap water.

If the water change alone doesn't raise and stabilize the pH I will recommend running crushed coral. You really aren't going to add too much. It is only going to raise it so much and no more. I can''t tell you what that number will be because it depends on the chemistry of your water. Just start out with about half a cup of it in a media bag. If you don't rinse it off before putting it in the filter it will raise the pH quickly. Not a problem since this is a fishless cycle. If you rinse it first it will take a few days. Once fish are in this tank if you need to continue using it be sure to rinse before replenishing what's in the filter to prevent spikes.

It works by very slowly dissolving so give it time. If you find you still need it after the cycle is complete. you will have to add more from time to time. I run it in my filters constantly. I keep an eye on my pH level by running the test every month or so. If I see it has dropped from a constant 7.2 I know the CC needs attention. I actually run chunks of coral instead of the aragonite sand. When my pH drops I pull the bag of CC out and clean it. I usually add another piece to replace what has dissolved.

I also have some of the aragonite sand. If using it I would just occasionally replace it instead of cleaning it. If it is cleaned and put back in it tends to cloud the water for a little while. It doesn't hurt anything and settles fairly quick but I still replace it.

 

Welcome to Fishlore

Water changes alone may be all you need. Since it had been 25 days since you set the tank up the cycling process had probably depleted the original minerals present in you tap water. Change out at least 75% of the water and the water you are replacing should get the minerals back up and the pH stabilized to the level of your tap water.

If the water change alone doesn't raise and stabilize the pH I will recommend running crushed coral. You are really aren't going to add too much. It is only going to raise it so much and no more. I can''t tell you what that number will be because it depend on the chemistry of your water. Just start out with about half a cup of it in a media bag. If you don't rinse it off before putting it in the filter it will raise the pH quickly. Not a problem since this is a fishless cycle. If you rinse it first it will take a few days. Once fish are in this tank if you need to continue using it be sure to rinse before replenishing what's in the filter to prevent spikes.

It works by very slowly dissolving so give it time. If you find you still need it after the cycle is complete. you will have to add more from time to time. I run it in my filters constantly. I keep an eye on my pH level by running the test every month or so. If I see it has dropped for the constant 7.2 I know the CC needs attention. I actually run chunks of coral instead of the aragonite sand. When my pH drops I pull the bag of CC out and clean it. I usually add another piece to replace what has dissolved.

I also have some of the aragonite sand. If using it I would just occasionally replace it instead of cleaning it. If it is cleaned and put back in it tends to cloud the water for a little while. It doesn't hurt anything and settles fairly quick but I still replace it.

Thanks. That's very helpful. I'll be getting some crushed coral today. I was thinking of hanging a media bag at the back of the tank in the water flow from the pump. Would you recommend that?

 

Thanks. That's very helpful. I'll be getting some crushed coral today. I was thinking of hanging a media bag at the back of the tank in the water flow from the pump. Would you recommend that?

You are very welcome. Yes, that should work almost as well as actually having it in your filter.

 

You are very welcome. Yes, that should work almost as well as actually having it in your filter.

Just an update. Five days ago, when the pH was reading 6.5 and KH 3, I added a 45g bag of crushed coral. The pH and KH continued to drop and yesterday, with pH 6 and KH <1, it looked as though the cycle had stalled again. I no longer see the point in keeping it going with short term fixes (bicarb), so I added another 80g of crushed coral and did a 20% water change. Today the pH is still 6 and KH still <1.

Nothing seems to raise KH for any more than a day or two. Have I just got some freakish water chemistry that can't be fixed? Will the cycle continue to stall with fish, and will any fish tolerate such a low pH and KH, or should I just give up?

 

Please don't give up

Don't be afraid to do big water changes. I would change out at least 75% or even 80% of it. The bacteria isn't free floating in the water so doing a water change isn't removing any. As long as you add your water conditioner to the fresh water it can only help. Since it seems the minerals are low in your tap water it is going to take a much bigger water changes to replenish them. Once this cycle is done the crushed coral should hold your pH up to or very close to the pH of your tap water.

Once the cycle is complete your weekly water changes should keep things stable. My water is very soft and is very low in minerals. I have to run crushed coral in my tanks and use seashells in my decor.. I also add Equilibrium to my tanks. The gh/kh tests are difficult for me to read so instead of running those tests I use my TDS (Total Dissolved Solids) meter. May not be the right way but is what works for me. The TDS from the tap is 21. I get it up to 100 in my fish only tanks and 150 in my shrimp bowl with the Equilibrium.

 

Thanks. The problem is that nothing seems to raise the KH in a stable way over the longer term. The crushed coral has been in there for 6 days now. When I added it (unwashed) the KH was 3 (due to adding potassium bicarb) but it quickly dropped back down to <1, where it is now. As the tap water KH is <1 I can't see how water changes would make a difference. I was looking at Seachem Alkaline Buffer - is that something I could add to weekly water changes that would keep the pH/KH stable through the week?

 

Seachem Alkaline Buffer

Chemically that's the same as bicarbonate. Just more expensive because it's by Seachem. Save the money.

You have different bacteria that live at a PH of 7 than a PH of 6. You can cycle your tank at either PH but not both.
Constantly chasing these water parameters is not a game that you want to play.
At some point you will misdose overdose or forget to dose and everything comes crashing down.

I agree to the fullest.

After all of this, billyP may I ask what fish you are even aiming for? Because there is a broad spectrum of animals you can keep in your water unaltered and it is possible to cycle your tank in that low pH/KH, but then it's a completely different set of microorganisms.

Your water is PERFECT for blackwater and clearwater species. Many popular species are among those. Using tannins and humic acids you would also not have to worry about sudden pH drops as the pH will then stabilize between 5.5. and 6.5 and not be bad.

 

Chemically that's the same as bicarbonate. Just more expensive because it's by Seachem. Save the money.


I agree to the fullest.

After all of this, billyP may I ask what fish you are even aiming for? Because there is a broad spectrum of animals you can keep in your water unaltered and it is possible to cycle your tank in that low pH/KH, but then it's a completely different set of microorganisms.

Your water is PERFECT for blackwater and clearwater species. Many popular species are among those. Using tannins and humic acids you would also not have to worry about sudden pH drops as the pH will then stabilize between 5.5. and 6.5 and not be bad.

Thanks for the response. The whole idea of the tank was initially something for the kids to get involved with, so I was planning on keeping hardy, adaptable fish like zebra danio. Would they be comfortable in this environment? If not, could you recommend any good alternatives?

 

I'd go with tetras: cardinals, embers, lemons, x-rays, silver tips, glow lights.

10 of one of these species should do good in those water parameters.

But as I said, you will have to add a relatively high amount of tannins to keep the (albeit low) pH stable.

How does the tank look right now?

 

Just out of curiosity, what "inert" substrate are you using specifically? quartz sand? some form of aquasoil?

 

I'd go with tetras: cardinals, embers, lemons, x-rays, silver tips, glow lights.

10 of one of these species should do good in those water parameters.

But as I said, you will have to add a relatively high amount of tannins to keep the (albeit low) pH stable.

How does the tank look right now?

I dosed ammonia up to 2ppm a couple of days ago and it's now down to 1 (when I was keeping the pH up with bicarb it would process 2ppm in 24 hours). NO2 0.75 / NO3 40 / pH 6 / KH <1 /GH 12. I'm running it at 23 degrees C. Plants are thriving and water's clear, with no algae problems. What method of adding tannins would you recommend?

Just out of curiosity, what "inert" substrate are you using specifically? quartz sand? some form of aquasoil?

I'm using Tropica substrate and gravel.

 

I dosed ammonia up to 2ppm a couple of days ago and it's now down to 1 (when I was keeping the pH up with bicarb it would process 2ppm in 24 hours). NO2 0.75 / NO3 40 / pH 6 / KH <1 /GH 12. I'm running it at 23 degrees C. Plants are thriving and water's clear, with no algae problems. What method of adding tannins would you recommend?

I'm using Tropica substrate and gravel.

That explains quite a bit...especially if we ended up with their plant soil

From the tropica website, Aquarium Soil - a perfect, complete bottom layer for aquariums - Tropica Aquarium Plants

"Aquarium Soil is further an active bottom layer that lowers the pH value and slightly affects the water chemistry. We recommend that you change 25-50% of the water min. twice a week during the first 4 weeks after establishing the aquarium."

 

Just out of curiosity, what "inert" substrate are you using specifically? quartz sand? some form of aquasoil?

I'm a bit off lately, how could I forget to ask.

I dosed ammonia up to 2ppm a couple of days ago and it's now down to 1 (when I was keeping the pH up with bicarb it would process 2ppm in 24 hours). NO2 0.75 / NO3 40 / pH 6 / KH <1 /GH 12. I'm running it at 23 degrees C. Plants are thriving and water's clear, with no algae problems.

When I asked how the tank looks I was implying I would like to see a picture.
NO3 at 40 is high.

What method of adding tannins would you recommend?

Get your actual water parameters in order first, before using any of these:
- Leaf litter
- botanicals
- peat
(or all of the above together)
Be aware tannins colour the water brown like tea.

 

That explains quite a bit...especially if we ended up with their plant soil

From the tropica website, Aquarium Soil - a perfect, complete bottom layer for aquariums - Tropica Aquarium Plants

"Aquarium Soil is further an active bottom layer that lowers the pH value and slightly affects the water chemistry. We recommend that you change 25-50% of the water min. twice a week during the first 4 weeks after establishing the aquarium."

No, it's not that one. It's Tropica plant growth substrate, which it says is neutral and doesn't affect KH/pH.

I'm a bit off lately, how could I forget to ask.


When I asked how the tank looks I was implying I would like to see a picture.
NO3 at 40 is high.

Get your actual water parameters in order first, before using any of these:
- Leaf litter
- botanicals
- peat
(or all of the above together)
Be aware tannins colour the water brown like tea.

This is how it's looking.

 

Allright, you can think about the idea of adding tannins, BUT in dark water light only penetrates to a certain level, so you will see some plants melting, which is a for some parts a shame, but it's either tannins (least resistance) or using what feels like a ton of crushed coral, limestone or bicarbonate .

As other people (like me) have to get an RO unit to get your hardness levels for our fish, I really recommend going the route of a planted botanical style/blackwater tank, this means the least effort in maintenance and the least upkeep cost as botanicals are generally cheap, you can also just collect leaf litter (like oak) yourself in the woods. A bag of 25-50 liters should get you through until next fall and if it doesn't you can still get IALs and alder cones for 1-2 months to bridge the time left after you used up the leaf litter.

It also gets you far closer to ideal conditions for many fish.

No, it's not that one. It's Tropica plant growth substrate, which it says is neutral and doesn't affect KH/pH.

With the original tapwater reading of KH <1° I think the substrate is pretty much irrelevant.

 

Allright, you can think about the idea of adding tannins, BUT in dark water light only penetrates to a certain level, so you will see some plants melting, which is a for some parts a shame, but it's either tannins (least resistance) or using what feels like a ton of crushed coral, limestone or bicarbonate .

As other people (like me) have to get an RO unit to get your hardness levels for our fish, I really recommend going the route of a planted botanical style/blackwater tank, this means the least effort in maintenance and the least upkeep cost as botanicals are generally cheap, you can also just collect leaf litter (like oak) yourself in the woods. A bag of 25-50 liters should get you through until next fall and if it doesn't you can still get IALs and alder cones for 1-2 months to bridge the time left after you used up the leaf litter.

It also gets you far closer to ideal conditions for many fish.


With the original tapwater reading of KH <1° I think the substrate is pretty much irrelevant.

Thanks. I'm all for the path of least resistance!
A few more questions -

My test kit only reads as low as pH 6, so the tank water may be lower than that anyway. Is that a problem?

I know it's impossible to be precise, but roughly how much leaf litter should I add to a 60l tank, and what should I expect it to do to the water chemistry?

Would you add the leaf litter in a media bag?

 

My test kit only reads as low as pH 6, so the tank water may be lower than that anyway. Is that a problem?

It can be a problem. Though to be honest: You will notice from the fishes behaviours if something is wrong. I would only monitor during the first month or so, after that a bi-weekly check is usually enough.
There are wide range test kits by Fluval and by JBL (both range from pH 3 to 10), they should be available to you via Amazon.

I know it's impossible to be precise, but roughly how much leaf litter should I add to a 60l tank, and what should I expect it to do to the water chemistry?

You're right, that's hard to estimate. I have added about 3-5 leaves (Oak, beech, wild cherry, Catappa) every 2-3 weeks for over a year now.
Besides the low pH, you might see a minimal rise in TDS (provided you test for this. It stands for total dissolved solids) and a certain tint in the water ranging from yellow to a tea colour. Hardness (GH/KH) will stay below detection.
If there is any ammonia (NH3), under a pH of 7.2 it will be present as basically harmless Ammonium (NH4). Nitrite will likely not turn up anymore, as the bacteria doing the cycle in these conditions skip the Nitrite stage and metabolise Ammonium directly to Nitrate.

Would you add the leaf litter in a media bag?

No, I utilize leaf litter as a secondary food source, so I leave it lay on the substrate and rot, giving the fish access to it. The microorganisms on the leaf litter could sustain my fish for at least 2 weeks if need be. It also has structural properties and can be included in the hardscape. It can even be an integral part of it.

 

It can be a problem. Though to be honest: You will notice from the fishes behaviours if something is wrong. I would only monitor during the first month or so, after that a bi-weekly check is usually enough.
There are wide range test kits by Fluval and by JBL (both range from pH 3 to 10), they should be available to you via Amazon.


You're right, that's hard to estimate. I have added about 3-5 leaves (Oak, beech, wild cherry, Catappa) every 2-3 weeks for over a year now.
Besides the low pH, you might see a minimal rise in TDS (provided you test for this. It stands for total dissolved solids) and a certain tint in the water ranging from yellow to a tea colour. Hardness (GH/KH) will stay below detection.
If there is any ammonia (NH3), under a pH of 7.2 it will be present as basically harmless Ammonium (NH4). Nitrite will likely not turn up anymore, as the bacteria doing the cycle in these conditions skip the Nitrite stage and metabolise Ammonium directly to Nitrate.


No, I utilize leaf litter as a secondary food source, so I leave it lay on the substrate and rot, giving the fish access to it. The microorganisms on the leaf litter could sustain my fish for at least 2 weeks if need be. It also has structural properties and can be included in the hardscape. It can even be an integral part of it.

Thanks. That's very helpful. I got a bag of dead oak leaves from the woods yesterday, and I'll look at the low pH test kits. I'm looking at various tetra and I'll choose whichever suits the tank best.

A few more questions!

- My understanding is that tannins will lower the pH slightly. Is that a risk, being at such a low pH already, or will they help stabilise it?

- While I was regularly raising the pH with bicarb the tank was processing 2ppm ammonia in 24 hours. Since I started to let the pH stay at 6 (or below?) it's taken 72 hours to get to zero ammonia. As you say that below 7.2 pH the ammonia is present as harmless ammonium, and I'm no longer expecting the pH to rise, should I consider the cycle complete? (current readings before weekly 20% water change - NH3 1 / NO2 0 / NO3 20).

- As the crushed coral seems to be having no real effect, other than raising the GH from about 8-10 to 12, should I just remove it?

 

I got a bag of dead oak leaves from the woods yesterday

Remember to sort and dry them. Mold can ruin the whole bag.

- My understanding is that tannins will lower the pH slightly. Is that a risk, being at such a low pH already, or will they help stabilise it?

They help stabilize. That's why I emphasized adding them. Add a good amount (handful) of leaf litter immediately and add the same amount weekly over 2 weeks with the waterchanges. Oak leaves don't colour the water as much, but they are great for stabilizing pH. Leave them in until they are mulm and let the mulm do it's thing. Doesn't look nice, but this is how you fuel an ecosystem. With leaf litter you don't need to feed as much, the fish find food betwen the leaves.

- While I was regularly raising the pH with bicarb the tank was processing 2ppm ammonia in 24 hours. Since I started to let the pH stay at 6 (or below?) it's taken 72 hours to get to zero ammonia. As you say that below 7.2 pH the ammonia is present as harmless ammonium, and I'm no longer expecting the pH to rise, should I consider the cycle complete? (current readings before weekly 20% water change - NH3 1 / NO2 0 / NO3 20).

Pretty much. The low-pH-cycling (The composition of the microfauna in your filter has to adjust as different species take over the cycle) will take some months, but it will still be working. As you are only going to stock lightly and have plenty of plants I see no problems ahead. Still keep an eye on it.
Before adding the Tannins, maybe do a 50% waterchange.

- As the crushed coral seems to be having no real effect, other than raising the GH from about 8-10 to 12, should I just remove it?

Yes, you can remove it.

 

Remember to sort and dry them. Mold can ruin the whole bag.


They help stabilize. That's why I emphasized adding them. Add a good amount (handful) of leaf litter immediately and add the same amount weekly over 2 weeks with the waterchanges. Oak leaves don't colour the water as much, but they are great for stabilizing pH. Leave them in until they are mulm and let the mulm do it's thing. Doesn't look nice, but this is how you fuel an ecosystem. With leaf litter you don't need to feed as much, the fish find food betwen the leaves.


Pretty much. The low-pH-cycling (The composition of the microfauna in your filter has to adjust as different species take over the cycle) will take some months, but it will still be working. As you are only going to stock lightly and have plenty of plants I see no problems ahead. Still keep an eye on it.
Before adding the Tannins, maybe do a 50% waterchange.


Yes, you can remove it.

That's all reassuring! Thankyou.

How long would you suggest I wait between adding the leaves and adding the fish?

With the leaves forming mulm, how should I approach syphon-cleaning of the substrate?

 

How long would you suggest I wait between adding the leaves and adding the fish?

With the first, bigger load of leaves wait a week, as the bacteria use up a lot of oxygen while colonizing the botanicals. As soon as the initial depletion is over it's safe to add fish and just a handful every few weeks is also no problem.

Otherwise I can only encourage you to get alder cones and brew up some extract for every waterchange, as the leaves alone may not be enough, but you can't just add tons of botanicals.

With the leaves forming mulm, how should I approach syphon-cleaning of the substrate?

Not at all. I haven't siphoned the substrate in my tanks in years. This only works because of the kind of mulm. Many people have a high stocking density and feed what feels like metric tons of food, so their mulm consists mainly of leftover foods and fish waste. That type of mulm is high in nutrients and energy, which leads to foulness and can cause ammonia spikes. Also if the fish brought in parasites this is where those accumulate.
But with mainly botanical mulm from dead leaves this is not a problem. The trees retract almost all nutrients from the leaves before shedding them. So the mulm is relatively poor in nutrients and microorganisms from bacteria up to infusoria, worms and crustaceans can handle the amounts easily.

As you are only about to stock a small group of small tetras there is little waste that accumulates and the microfauna will deal with it.