29 Gallon Tank PH Crashed and High Nitrites

  • #1
Hey guys!

I have a pretty heavily planted 29 gallon that has been up and running since January 2020. 2 weeks ago I lost a Bolivian ram and a black phantom tetra within a couple of days of each other. I attributed it to a pathogen that got into my tank in summer 2022 after I added a shoal of corydoras. The symptoms were the same as the handful of fish I lost then. When I checked the water after losing the fish, I found that the PH was 6.4, ammonia & nitrite zeroed out, and nitrate between 5 & 10. As it normally sits in the 6.6-6.8 range I didn't think much of it. I did a 50% water change and started a salt treatment with standard aquarium rock salt. I've been hesitant to use medications after using them so heavily through the summer. I do not want to over treat and wind up with immunoresistant pathogens in my tank.

The first week I treated at 1 tablespoon per 3 gallons. After the first week I ran another test before changing 30% of the water and upping the salt content to 1 tablespoon per 2 gallons. The results were concerning. The PH had dropped significantly. Here's how it read:

API Master Kit:

PH 6.0
Ammonia: 0.0 ppm
Nitrite: 0.0 ppm
Nitrate: 20

API Hardness Strips:

GH: 60-120 ppm (increased significantly from the norm due to salt treatment)
KH: 0-20 ppm

The color of the fluid in the PH vial was the same yellow as a 0.0 ammonia test, completely bottomed out. I reran this test several times to confirm I had done it correctly.

I took a sample to my LFS and had them check my parameters. Their results were very similar except they registered extremely high nitrites. We chalked this up to a contaminant in the vials as none of my repeated tests prior to bringing the sample had read anything other than 0.0. We agreed that I needed to raise my KH to stabilize my PH. So during the water change, I replenished the crushed coral I normally keep in my filter as a decent amount had dissolved since the last time I refilled. I added around triple the normal amount.

This past week I have had 1 tablespoon of salt per 2 gallons of water in the tank. When I tested today before changing the water, I was shocked with what I found. The PH had not increased one bit. It was still completely bottomed out at 6.0 and I had registered nitrites in the water for the first time in almost 3 years. Here is what the first test registered:

PH: 0.0
Ammonia: .25 ppm (can be hard to tell on the API kits when its a 0.0 or a .25)
Nitrite: 1.0 ppm
Nitrate: 0.0 ppm

I re-ran this test several times and in about half of the tests I registered a sky blue, 0.0 on nitrites and 0.0 ammonia and the other half purple .5-1 ppm nitrites. Since I was not getting consistent results, I went to the PetSmart around the corner and bought another API kit in case the vials or drops themselves had been contaminated. The test with the brand new kit read like this:

PH: 6.0
Ammonia: 0.0 ppm
Nitrite: .5 ppm
Nitrate: 0.0

Obviously this is a problem. My nitrogen cycle is sliding off the rails. I have read that the nitrifying bacteria responsible for processing nitrites into nitrates isn't able to do so at PH's less than 6. So that seems to be the issue behind the nitrites.

I am absolutely perplexed by the PH. I typically change water every 2 weeks or so and the PH stays rock solid at the 6.6-6.8 range. Prior to the fish deaths it had been 3-4 weeks due to a series of family emergencies that kept me out of town for almost all of the month of December. Obviously, not ideal to wait that long, but I would not expect that to lead to a total crash of the PH. Seeing as my tap water is usually 6.8-7.2 I would have expected a 50% water change and a 30% change a week later to rectify this, especially considering the crushed coral I introduced. The only variable I can think of has been some fluctuations in the tank's temperature. It is beside a large, not well insulated window and I have been struggling to keep the temp dialed in at the same number as its gotten colder. It has moved between 75-80 degrees as I've tried to keep up with the weather outdoors. I have also started pulling filter floss from a new bag of poly-fill. However, it is the exact same product as the previous one. No additives, 100% inert polyester.

Everything I have read says that salt treatment with regular old sodium chloride will not impact PH levels (GH will increase of course). I am wondering at this point if the uptick in plant decay (ammonia) due to the stress of the salt treatment is playing a part. I am going to dose prime to detoxify the nitrite and do daily water changes, but until I can get the PH back up to a manageable level this is only going to treat the symptoms not fix he actual issue. Once we get the PH moving back up I am going to dump a bottle of Dr. Tim's bacteria in a bottle into the filter as well.

If anyone has any insight or advice on how to get things back on track I would greatly appreciate it.

Frank the Fish guy
  • #2
You have no buffers in the water. Buffers are KH. You have not tested KH yet, but if you do it will probably register zero.

The cycle will stall abruptly when the pH falls to 6.0 since you have grown a biofilter for neutral water. these bacteria will stop at pH of 6.0.

You can put some baking soda in the water to raise KH immediately so that your cycle stall won't poison your fish. KH=3 would be a good goal to get your cycle back up and going. Get your pH above 6.0 and back to where it was before.

Whatever crushed coral you are using in your filter is not really calcium carbonate and is not working.

Rock salt does not increase KH, only GH and salinity. Baking soda is what you need to boost your KH.
At pH of 6.0 the nitrites you have are deadly.

Nitrites are fatally toxic at low pH any anything measurable is fatal at a pH of 6.0. So this is an emergency situation where you must raise the pH quickly to keep the nitrites from killing your fish.

Salt can help with nitrite toxicity, but not in this case.

  • Thread Starter
  • #3
I appreciate the response. From what you are describing it sounds like I probably let it go too long before adding more coral to the filter and was too little too late when I finally added more. I usually don't put very much in it as I keep fish that are suited for the soft water that comes out of my tap.

I did a 50% water change this evening and have a second mesh bag filled to the brim sitting directly in front of my filter's out flow. Tomorrow morning I will head out to grab some baking soda and pick up some additional hardness strips. What would you suggest as the dose to bring the KH up?

Edit: Looks like we had more baking soda than just the pack stuck in the back of the fridge for odors. My wife had some buried in the back of the pantry.
Frank the Fish guy
  • #4
The rule is 1/4 tsp of baking soda raises the KH by 12 degrees in 1 gallon of water.

So for your 29 gallon tank, to raise KH about 3 degrees, use about 1.75 teaspoons of baking soda.
  • Thread Starter
  • #5
Gotcha. My PH after the water change was 6.6. Along with the crushed coral I added a quarter tea spoon last night and another quarter teaspoon this morning. Seeing as it was late, I didn't want add so much to shock them while I was asleep.

I just checked the PH and its somewhere between a 6.4 and a 6.6. Still have some nitrites, so I will do another water change then head to LFS for the hardness strips. Once, I have those I should be testing after each change to see how much baking soda I need to get to 4 dGH?
Frank the Fish guy
  • #6
Right. You want a KH test. 3-4 dKH is a good goal to get you going again.

The pH swings don't really shock the fish directly, what shocks them is that nitrite is more toxic at low pH.

At a pH of 6.6, nitrite levels of .5 ppm are toxic to fish.

The following may be an 'advanced' technique, but honestly what you may need to do.

You can temporarily raise the pH to 7.4 to detoxify the nitrite. The toxic level at a pH of 7.4 is about 2 pmm. The pH change will not harm the fish. What harms the fish is the nitrites at low pH.

If your crushed coral is big fat chunks it may not be dissolving. Take a hammer to it and grind it up into a powder. Or use egg shell, or oyster shell. The whole point of calcium carbonate in your tank is to prevent what happened to you. ;)

Remember that 1 degree of KH is equal to about 17 parts per million (ppm).

So when we say to raise the KH we want to get to 3 degrees KH. That is equivalent to about 50 ppm. That is still soft water.

When you used your test strips the first color is 0-20 ppm, which was 0 ppm for you.


  • #7
Yeah, it requires carbonic acid and nitric acid to be created in tank to cause the crushed coral to erode and release calcium and carbonates to buffer. I would think your KH is depleted, but not exactly sure what's going on.
When it happened to me it went completely unnoticed, what tipped me off was new fish would die, fish I had did fine, new fish couldn't acclimate. I resigned myself to just keeping the fish i had and stoped trying to add new ones. However 2022, when I went to correct it. It took forever for anything to budge, GH was high KH was non-existent, pH was 6 and lower, my nitrates were huge.

The point is though after months of water changes slow and steady to dig out nitrates, and bring down GH, I still had low pH and zero KH. pH and KH started rising months of water changes later. Took about a year to dig it out without a full tear down and just doing it gradually.

Now, what happened. When my KH returned, and my pH started to rise again, I got nitrites. I mean like 4ppm nitrites, but the fish were fine i was just into 6.2 6.4 pH when it happened and i dead stopped water changes. It took 3 weeks for the cycle to catch up,,, or transition from low pH.

I think that also happened on my way down, I just wasn't monitoring to see it, but there was a massive pile of nitrates as an end result of that KH depletion and the pH dropping out and doing nothing about it and just letting it ride.

I think, like me, it's Old Tank Syndrome, you just happened to catch it dropping out because of the deaths, with mine, nothing died and I wasn't adding new fish at the time, that came later and no matter what I tried, new fish wouldn't acclimate to it.

There's plenty of acids in the tank, this is going to burn your new buffer right up and continue to until the buffer can cancel it all out. It's why it took me like a year to dig it out and why even after nitrates were gone and GH was down, I couldn't touch KH or pH for another few months of water changes.

I wish you luck.
  • #8
The highest nitrate reading has been 20ppm so I think it might be a case of very soft source water rather than old tank syndrome.

Are you on a city water supply? What are your tap water parameters, especially pH, KH & GH? If the issue is with your tap water I think you're on the right path to deal with it.
  • Thread Starter
  • #9
Thanks for the feed back guys! While I was out this afternoon I picked up some API KH and GH drops. Seemed more economical than buying packs of 4 strips every time I needed them. I tested my tank again and got this:

PH <6.0
Ammonia: 0.0
Nitrite: 2.0 (yikes)
Nitrate: 0-5.0 (kinda in the middle)
KH: 0 dGH
GH 5 dGH

Our municipal supply is very soft here. I've known that and tried to keep fish in alignment with this. I set out a glass of tap water last night and just ran a test on it. Came out like this:

PH: 7
Ammonia 0.0
Nitrite: 0.0
Nitrate: 0.0
KH: 2 dGH
GH: 2 dGH

Given this I am more inclined to agree with GlennO. Or at least I'd like to. However, it should be noted that my tank is heavily planted so I've rarely seen my nitrates get over 20ppm. I'm not sure if it would be possible to enter "Old Tank Syndrome" while my plants are eating up the nitrates.

Though it could be a possibility, I have had trouble acclimating fish on the past. As I work through this, a part of me wonders if this was due to the low KH levels. Tbh, I have checked these very rarely while doing the standard 4 on a very regular basis.

Would anyone happen to have any resources they could direct me to regarding the mechanisms of how KH impacts PH? I'd be interested in reading about the chemistry of it.
Frank the Fish guy
  • #10
Your tank has depleted the buffers again. The cycle has stopped. The cycle uses KH up.

I have water like yours. I use soda ash to buffer my water. Soda ash is the next step up from baking soda. It is a stronger and more permanent buffer. Soda ash is the ash left over when you heat baking soda. You can make it yourself by heating baking soda, or just buy it. Soda ash is sodium carbonate.

You started this tank in 2020, so not an old tank. Just no buffers because of your low KH source water.

This tank water is deadly to fish.


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  • #11
This morning I did a 50% change with a dose of baking soda and did 100% over the last 2 hours. I added 1.5 tsp of baking soda after finishing the last one several minutes ago. Hopefully, this will hold the PH up overnight. I'll check again before work and swap water/dose baking soda as needed.

As a long term solution, would the path forward be to include baking soda/soda ash as a part of every water change in addition to keeping the crushed coral in the filter?
Frank the Fish guy
  • #12
Yes that is what I would do. And use soda ash going forward as your buffer and get KH up to your goal and it will be long lasting.

I use a chemical feed pump and I raise my KH up to 8 dKH actually.

I think your water and tank is not a candidate for mere crushed coral. You need to use soda ash to buffer the source water.

It is a matter of rates. The cycle uses the carbonate faster than your crushed coral puts it in. So it crashes.

Note that you are not raising the GH so the water is still soft for your soft water fish.
  • Thread Starter
  • #13
Yeah, I certainly don't want to wind up in this position again. So, I am trying to get a good understanding around the relationship between the nitrogen cycle, PH, and KH.

Tell me if my understanding is incorrect. Acids are released as a by product of the nitrogen cycle and these acids need to bind to an alkaline like sodium carbonate to neutralize. If the alkalines are depleted then the acids will have nothing to bind with and crash the PH, once the PH crashes below 6 nitrifying bacteria start to die?

Along the lines of nitrite toxicty, that is related to PH and temperature like ammonia toxicity? I know with ammonia toxcity you actually have less free ammonia at PH's in the 6's vs. 7's. But from you are describing it sounds like nitrite might be the opposite? Do you have any links that break that down a bit? I'd love to read into it. Obviously, you want to keep your parameters zero'd out in this regard, but knowledge is power as they say.
Frank the Fish guy
  • #14
Yes, nitrite is the opposite to ammonia with regards to its toxicity vs pH.
Nitrite - The Free Freshwater and Saltwater Aquarium Encyclopedia Anyone Can Edit - The Aquarium Wiki

OK, check out blackwater aquariums too as a different approach to your situation.

In a blackwater, acidic tank you remove any calcium carbonate (crushed coral, etc) and don't add any KH buffers. You let the KH in your water get absorbed by the acids in the tank.

The tank becomes acidic with a low pH and the decaying plants and leaves become the buffer. This is an acidic buffer.

You have to grow a different kind of bacteria for this environment, and that will happen naturally and just take time, like re-cyclng.


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  • #15
Good news! This morning I checked the PH and it held at 6.8 overnight and had not built up any ammonia or nitrites.

This evening I tested everything and got this:

PH 6.8
Ammonia 0.0
Nitrite: 0.0!!!
Nitrate: 0.0

KH: 1 dGH
GH: 2 dGH

I tested everything twice to be sure.

I had not fed the fish since Saturday. With the amount of decaying organic material present in the form of plant matter and the mulm that I can't get to in the most densely planted areas I am optimistic that we will start to see them soon. I just fed them, so the real test will come over the next hours/day.

I am going to continue to change water and add soda ash as I go to build up a larger KH buffer. Once I build up that buffer and am confident that I will not have to change water every 12-24 hours I am going to toss a large bottle of Dr. Tim's bacteria into the filter.

I'm fascinated by blackwater tanks. I think the aesthetic is very cool. However, I did not realize you had to approach it from a different angle in regards to the nitrogen cycle and mineral content. Definitely something to consider in the future, I'd love to set something up with that aesthetic. But, I need to be able to manage the difficulties of my soft water supply in a traditional manner before jumping into that realm!
  • #16
Just saying, you can totally do the blackwater aesthetic without doing proper blackwater. Add a bunch of bontanicals which will release tannins, which make the water darker.
  • #17
Have you stopped using salt? It's use would be doing more harm than good especially if there are no nitrites present.
  • Thread Starter
  • #18
Yeah I haven't added any salt since last weekend. Since Saturday afternoon I've probably changed out 300-350% of the water. At this the point the GH reading is coming from just what is in my tap water.

I'm going to focus on getting this parameter locked in and then re-evaluate for signs of infection. It seems far more likely to me that the issue over the last few weeks has been an unstable PH rather than an infection.
Frank the Fish guy
  • #19
Good to see your KH is holding above zero. Get the KH up to your goal to build a buffer to avoid future crashes.

The pH will go up as you raise the KH to your goal, but it's the KH that is the key for you. Let the pH just 'come along for the ride'.

There is no problem with a variable pH actually, the problem is when the pH hits 6.0 and the cycle stops as you have seen! Natural waters and many tanks too have highly variable pH during the 24-hour day.

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