Overnight nitrate crash

I’ve had an interesting couple of days!
Found a dead fish yesterday morning and the investigation began to figure out why. The resulting water tests have shown the nitrates in the system dropping from 160ppm to 20ppm within 24 hours, and I know that such a dramatic change can be stressful for fish. The ammonia and nitrites read as 0 as always.

Water tests yesterday showed that the GH was high and the pH was creeping up, so I carried out a 15% water change. My treated tap water is pH 7.

Since then I have continued to feed the fish. I have also removed a potato plant and added five tomato plants to the growbed, but surely you wouldn’t see that fast a change?

I share this experience as something that’s generally interesting, but if anyone with better knowledge of the biochemistry involved in such a dramatic change I’d really appreciate the insight.


For a few months I’ve struggled to get the pH to stay above 6.2 in my little backyard system. No matter what I added or changed the carbonate hardness would go back down and the pH would return to 6.2 within a couple of days, although the GH was creeping up the whole time. A breakthrough was adding a bag of shell fragments as a buffer (although the jump in pH was faster than I would have liked.) I removed the bag after three weeks when the pH reached about 7.2.


Thanks for sharing about your experience. Just curious, the nitrate tests was this just from a test kit 24 hrs apart? We saw some fairly large day to day changes on our system in Jordan even though everything else remained pretty consistent.

Are you continuously monitoring other things in your system (like with probes)?

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Yeah, just a manual test kit at the moment.

Good to hear that large fluctuations aren’t unusual though.

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Hi Hana,
You may want to consider sending your water to get tested - usually about $40. GH test is good for a rough idea on your carbonate levels but there might be other things affecting it. What are you using to adjust your carbonate levels (besides shell fragments)? Are you using grow beds? If so, it’s possible you stirred up your bacteria colonies when you pulled the potato plant and that might have caused a rapid grow in bacteria that consumed the nitrate. I have seen spikes and drops when pulling large plants out of grow bed systems.


You may be right there. The potato plant was growing directly in the clay media growbed, and the tomato plants added are in hessian-lined pots of soil in the same clay media growbed.

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