Bacteria keeps dying. Help/Advice

Background info I cycled the empty 23 gallon tank for a month before seeing Nitrates and added fish gradually. 10 Cherry shrimp, 13 neone tetras, one Beta and two Sword tails. I feed the fish twice a day. AMOSIJOY 172GPH Canister Filter, with a floss pad, carbon sponge, ceramic filter rings, and Bio balls.

My problem is that my bacteria seems to die every 2 weeks or so. I see my Nitrates fall to zero, then the ammonia starts to go up slowly. I was told that maybe my filter is doing too good of a job and starving the bacteria. Is that possible? I keep having to add bacteria to the tank and I am wondering if I should just add more fish to creat more waist. I think the ammonia spike caused the beta to get a touch of fin rot, I am currently treating him for it and he is doing well. Adding the Nitrate/Nitrite readings for this tank (left) and a shrimp tank (right)

TLDR: Bacteria dies off (I think it's being starved), should I add more bio load or change the filter to something else. Also, is there something that may be killing the bacteria?

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u/EphemeralAttention 8h ago

What are your KH and PH levels like? Nitrifying bacteria use carbonates as their primary carbon source, so if your KH is dropping to zero that can cause the cycle to stall. The bacteria also work best at pH levels between 7-8. They still process waste between 6-7, but if your PH drops below 6 the cycle will significantly slow down.

Edit: fixed typos

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u/Azedenkae PhD in Microbiology 7h ago

A lack of carbonates may only impact the cycling process specifically, and not once it is done though. This is because once established, the population of nitrifiers no longer really need to grow.

As for pH and temperature, it does depend on the specific type of nitrifiers that one happens to cultivate. Not all prefer a relatively higher pH/temperature. Some indeed prefers pH <7. There is even an archaean nitrifier that can only perform nitrification at a pH of 4-5.5.

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u/EphemeralAttention 6h ago

Interesting, and good to know. Do you know if the ammonia oxidizing bacteria are capable of utilizing NH3 as readily as NH4+?

Based on OP's stated PH reading this wouldn't be impacting their tank, but now you've got me wondering if low pH could still result in a bottleneck to the reduction of ammonia to nitrite as a result of the low PH shifting the balance of ammonia and ammonium in solution towards ammonia. I don't know enough about the metabolic pathways and catalysts involved to know if the bacteria can process ammonia directly if it's more readily available than ammonium.

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u/Azedenkae PhD in Microbiology 6h ago

pH swings can definitely cause nitrification to slow down or stop completely, although it is not in one particular direction.

This article is about a nitrite oxidizer, not ammonia oxidizer, but similar idea: https://pmc.ncbi.nlm.nih.gov/articles/PMC4784051/. Here, a freshwater nitrite oxidizer was identified that has an optimal pH 6.8. Which means that if the pH gets lower than that, nitrite oxidation by said species slows down. But if pH gets higher than that, nitrite oxidation also slows down.