Is it possible that no elevated nitrite levels can be detected a few days after a nitrite intoxication?
recently, 4 of my fish died suddenly. Now, two weeks after these deaths, I could at least check the water for nitrite. This revealed that there were 0.025 ppm nitrite in the water. Is it possible that nitrifying bacteria have developed within the last two weeks and lowered the formerly too high (and thus maybe fatal) nitrite level due to their metabolism?
yes, this is indeed possible. Dangerously high nitrite levels become lower again after a couple of days unless there are no chemicals in the water that inhibit bacteria. The so-called "nitrite peak" during the biological activation period usually does not last longer than a few days. You can avoid or alleviate it with sera nitrivec, ideally in combination with sera filter biostart.
I did not test for copper since there are plastic water pipes in the house and I did not want to purchse the expensive copper test kit. Is there maybe another possible explanation?
There are many possible causes for an intoxication, including plant protecting agents from gardens or agriculture (this occurs only rarely, but there have been confirmed cases). Later on, it is unfortunately difficult or even impossible to tell whether nitrite actually was the cause. I recommend always having a nitrite test kit at home (nitrite by far is the most common intoxication cause in aquariums), because it is advisable to check it immediately if you suspect it. This will allow taking appropriate counter measures without delay.
Dr. Bodo Schnell