Minimum kh for stable cycle

[Joshaeus]

Hi everyone! I recently discovered that my tap water has too little kh to support a stable nitrogen cycle, so I have started adding baking soda to my water change water. According to the rotala butterfly calculator, the amount I am adding (1/4 tsp per 4 gallons water) adds just under 2.5 degrees kh to the water...how much kh is required for a stable cycle? I am assuming 2.5 degrees isn't enough, and at least one of my tanks still has a ph too low for a stable nitrogen cycle to develop after two 40% water changes with this baking soda enriched water. Thanks

 

[bgarthe]

A constant 2.5 KH is going to stabilize your pH. The lower the number (2.5 as opposed to say 5) simply means the ability to deal with acids is shallower, meaning it will peter out/lose its buffering ability faster going to 0 where pH can become unstable. The key is KH stability at whatever number. My KH in my 8 tanks is between 2-5 as that is what my chosen fish seem to prefer. The WC new water is a constant KH, GH, and pH. Incidently, all my thoughts could turn around based on your posting the KH and GH and pH.
There are keepers who keep KH at 0, but I’m not among them.

A bag of crushed coral in a HOB outflow can keep KH decent as well.
Just curious……what is your pH and GH?
What kind of fish are you going to have?

 

[tinylostwine]

You can't have too little kh for a stable cycle. I run all my aquariums with 0 kh. There can be an issue with ph drops very low (some of mine are below 5) with the type of bacteria that forms - it gets complicated per sey.

Anyway before you start tweaking your water chemistry I would first figure out how you are going to stock your aquarium. If you are going to keep guppies then yea add some gh and kh - there are various ways of doing such but if you are going to keep cardinal tetra or panda cory don't bother adding anything to your water.

 

[ruud]

Sometimes I wonder if people should start the aquarium hobby with a blackwater tank. The RO/DI unit is probably the main barrier to entry, but other than that, the chemistry is just about keeping the TDS low.

Lots of apistogramma, discus, angelfish, tetras, pencilfish, corydoras, catfish, betta, parosphromenus, and rasboras to choose from.

The advanced can move on to community tanks, tap water, and fancy aquarium products.

I'm not even trying to be funny.

So what pH and KH are you trying to chase Joshaeus?

You already know you’ll get answers from experienced hobbyists saying they successfully run tanks anywhere from 0 dKH all the way up to Tanganyika levels.

If we collectively say dKH 5 and you run into (apparant) cycling issues, what's next?

 

[fishthatfit]

The number you want isn't a KH floor, it's a consumption rate, which is why you're getting three different answers in here and all of them are right.
Nitrification eats alkalinity as it runs, on the order of 7.1 mg/L as CaCO3 for every 1 mg/L of ammonia-nitrogen oxidised. One dKH is about 17.9 mg/L, so your 2.5 dKH is roughly 45 mg/L of buffer, which is somewhere near 6 ppm of ammonia processed before it is gone. Dose 2 ppm a day into a fishless cycle and that is three days to zero. Then pH falls, and nitrification slows under about 6.5 and roughly stops near 6.0. That is your stalled tank.
It also explains the tank that didn't come round after two 40% changes. Two of those only turn over about 64% of the water, so you diluted into a tank that is still burning buffer faster than you are putting it back.

And it reconciles tinylostwine with you, because you are both describing real tanks. An established tank at 0 KH is fine since its ammonia load is small and continuous, so the daily alkalinity draw is tiny. A cycling tank takes one big slug every day. Same water, different demand.

So rather than chase a KH number: add the baking soda to the tank itself instead of only to the change water, so you can top up mid-cycle; test KH as often as you test pH, since KH is the one that moves first; and drop the ammonia dose to 1-2 ppm. It cycles just as well and costs you a fraction of the buffer per day.

 

[ruud]

The number you want isn't a KH floor, it's a consumption rate, which is why you're getting three different answers in here and all of them are right.
Nitrification eats alkalinity as it runs, on the order of 7.1 mg/L as CaCO3 for every 1 mg/L of ammonia-nitrogen oxidised. One dKH is about 17.9 mg/L, so your 2.5 dKH is roughly 45 mg/L of buffer, which is somewhere near 6 ppm of ammonia processed before it is gone. Dose 2 ppm a day into a fishless cycle and that is three days to zero. Then pH falls, and nitrification slows under about 6.5 and roughly stops near 6.0. That is your stalled tank.
It also explains the tank that didn't come round after two 40% changes. Two of those only turn over about 64% of the water, so you diluted into a tank that is still burning buffer faster than you are putting it back.

And it reconciles tinylostwine with you, because you are both describing real tanks. An established tank at 0 KH is fine since its ammonia load is small and continuous, so the daily alkalinity draw is tiny. A cycling tank takes one big slug every day. Same water, different demand.

So rather than chase a KH number: add the baking soda to the tank itself instead of only to the change water, so you can top up mid-cycle; test KH as often as you test pH, since KH is the one that moves first; and drop the ammonia dose to 1-2 ppm. It cycles just as well and costs you a fraction of the buffer per day.

If you build a biofilter with high ammonia dosing and carbonate alkalinity, you select for a high-throughput, largely neutrophilic nitrifying community. Let the carbonate subsequently run out, and test-kit results may indeed look like a 'stalled cycle.' That tells you that particular dominant community has been pushed outside its operating conditions, but not that pH 6 is a universal biological limit for nitrification.

The microbial world is far more diverse and adaptable than that old wastewater-engineering pH rule, and the aquarium hobby that inherited it, suggests.

I wouldn't recommend adding baking soda to a hobbyist enjoying an Amazonian tank. Rather target and maintain a water profile that fits your fish or theme.

 

[fishthatfit]

Fair, and I stated it as a hard floor when it isn't one. Nitrification runs in acid forest soils and in low-pH filters, and the hobby did inherit that number from wastewater work rather than from anything about aquariums.

What I'd keep is the substrate side of it. Ammonia monooxygenase acts on free NH3, not on NH4+, and pH sets the split: at 25C and pH 7.5 roughly 1.8 percent of total ammonia is NH3, at pH 6.0 roughly 0.06 percent. Same 2 ppm on the test kit, about thirty times less of the molecule the enzyme can actually use. A community does adapt to that with higher-affinity organisms and more biomass, but it takes weeks at that pH to select for, and Joshaeus is mid-cycle with a pH that has already dropped. He's running the community that grew up under buffer at the substrate supply that came after it.

It cuts the other way on danger, which is worth him knowing. At pH 5.5 an ammonia reading is almost all ammonium, so the test result looks worse than the water is.

Your last paragraph is the one that should settle this thread, though. Joshaeus, bgarthe asked for your pH and GH and what you're stocking, tinylostwine asked the same thing, and nobody can give you a real answer until you say. Cardinals or apistos and ruud is right, leave the tap alone and let it sit where it sits. Guppies or shrimp and the buffer is worth adding, at which point the cycle question stops being a question.