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Nitrogen Cycle crash (rinse, lather, repeat)


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I have a 29 gal high with an AquaClear 50 HOB filter and an sponge filter that *was* established and doing awesome… until it wasn’t. It houses 13 cories (6 pandas, 5 albino, 2 emerald), a mystery snail, and a female betta.

I can’t pinpoint what caused the crash, but at some point my nitrogen cycle crashed with fish in the tank. I’ve lost 4 fish in two weeks because of high nitrITES. (I capitalized that on purpose) 

Filter media is:

course sponge,

crushed coral

Aquarium Co op bio rings/Fluval bio beads

and I had SeaChem Purigen in there, but I decided to switch it out for a piece of fine poly pad today since nothing else is working.

When this saga started, ph was very low, but thanks to some crushed coral, that’s come up.

Water parameters are:

Ammonia 0

pH 7.4 

nitrate 5

nitrite currently at 1.0, but if today is like all of the past 14, it will be back up near 4.0 by bedtime and I’ll spend the night berating myself for not doing a water change at midnight and hoping my fish are alive when I wake up. 

I’m going through SeaChem Stability at a ridiculous rate with no results.

Every day, I do my water check, do a 30% water change to bring the nitrites back down, dose with Fitz water conditioner and Stability, check parameters again (water change usually gets me to a nitrite level of 1.0) and by night it’s back up to danger level.

What am I doing wrong?


This tank was cycled fishless for almost 4 weeks, until parameters were spot on and stable for over a week. I added fish, everything was great for a month or two and then it stopped breaking down Nitrites. My water is RO water, it is a little soft, but has no nitrite reading alone.

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You could also try adding seachem prime because that should neutralize the nitrite directly. As for the cycle crashing, and the use of seachem stability. You might want to check to see if your stability has expired and if so then that may be why it isnt establishing. If you have any other tanks running, you could also try to take some of the bio media to help kick start the process. 

Quick question, did you add all of the fish at once or did you slowly add them in there over time? 

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Slowly over time. First it was just 6 emeralds, then I pulled the pandas out of another tank and moved them in a few weeks later. The pandas moved over next and betta just went in before all this crashing started.


I’ve been through 2 whole bottles of stability in the past few weeks trying to control this. So unless both were expired, I don’t think that’s it. And isn’t Fitz pretty much the same as Prime in regards to detoxifying?

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On 5/12/2022 at 3:03 PM, Onescout said:

use liquid CO2.

I’m not a chemist so don’t take this as anything more than I’m just throwing out something to think about.  Many liquid co2 products contain gluteraldehyde. This is harmful to humans and comes with human warnings. Some quantities are harmful to fish.  It is also used in cold sterilization where heat sterilization is not able to be used.  In other words it kills bacteria.  

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That’s a good thought. If that’s the cause, then maybe it’s a good thing that I’m really bad at remembering to add it very often, lol! But it’s possible that I’ve added it a few times since this all started!

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Oh my gosh, I just figured it out and feel like a total idiot!


we got the RO around the time the first crash happened. I thought “I tested the water, there’s no chlorine in it, so I won’t waste my water conditioner.” And completely forgot all the other benefits of water conditioner. Won’t make that mistake again.

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On 5/12/2022 at 1:27 PM, Onescout said:

And isn’t Fitz pretty much the same as Prime in regards to detoxifying?

I have not had the same results from both. 

On 5/12/2022 at 4:13 PM, Onescout said:

we got the RO around the time the first crash happened.

If you are not remineralizing, then the plants, bacteria and fish are missing necessary minerals to keep growth and stay healthy. Plus, RO eliminates kH. I mix my Pur water (typically 245 TDS, with ~120 ppm kH and 150 GH) with my ZeroWater to get down to 180 - 200 TDS and decent kH... but sometime between January and now, my water treatment facility has almost eliminated kH and my plants are showing magnesium deficiency, so I am having to do a lot more water testing and tryin to bring my kH back up....🤦🏼‍♂️

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You say the pH was very low and is now at 7.4. A very low pH inhibits the function of a biofilter. It's not a huge issue as ammonia and nitrites aren't very toxic at a low pH. When you raise the pH you increase the toxicity of the ammonia and the nitrites. Both are typically less toxic at a lower pH. The bacteria that convert ammonia to nitrites typically develop more quickly than the nitrite to nitrate converting bacteria. My suspicion is that your raising of the pH has increased the toxicity of the nitrites to the point where they became an issue. The biofilter is now adjusting to the higher pH and the necessary bacteria are present and growing, they just haven't reached a suitable population as of yet. Water is a complicated thing and doing something right (increasing KH and pH) can often have undesired consequences. In this case it made the existing nitrites more toxic and cost you some fish. The good news is your biofiltration should work more efficiently at the higher pH over time and the higher KH should stabilize the pH at that level. 

My best advice to pretty much anyone in the hobby is if what you're doing is working, don't go changing things. It's easy to read an article that says raising your pH and KH will make your tank more stable and assume you should do it, but if your ecosystem is adjusted to a lower pH, then it will need to adjust to the new parameters and that's not always a smooth adjustment. For now, just catch your breath, make water changes or use Prime to keep the nitrites under control, and things should settle down nicely. You have to be careful with water changes though as new water is likely to be more acidic and have less KH than the existing water. Crushed coral works slowly. You don't want wild swings. Good luck!

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