I've been doing some research on some scientific papers regarding the toxicity of nitrite to fish. Granted I haven't found many papers and there doesn't really seems to be any conclusive studies on this topic.
In the link I will provide the conclusions I have gathered are that nitrite toxicity is completely dependent on species and size of fish. Also amounts of chloride and bicarbonate present in the environment and levels of dissolved oxygen.
The article also describes how fish have a natural mechanism or reductase cells that reverse the formation of methemoglobin as it is accumulating. The lethality of nitrite is dependent on how well the species can do this. It also says that all fish have some levels of methemoglobin present in the cells as a percentage and that the starting percentage is dependent again on species.
There is also a chloride offset. My tap water report shows I have 5.85mg/l of chloride and I worked out that this would offset nitrite by around 2-2.5ppm according to this page.
If taking in to consideration the size of my fish and the species (impossible to study all species) and their background methemoglobin (percentage of meth already in cell) coupled with the ability of the species reductase cells to reverse the process. This could explain why some people have experienced elevated levels if nitrite with no consequence.
I'm still trying to figure out what levels of nitrite would be considered dangerous. This link seems to suggest nitrite levels of 1ppm upwards. But with everything else mentioned taken in to consideration I don't think it would come as a surprise to me that in some cases aquarium fish have survived higher concentrations.
I would always encourage people to change water in the event of nitrites and I'm not saying that it should be taken any less seriously. I was just wondering what your thoughts were and if you had any other papers that would provide an interesting read.
If fish in cycles are constantly reading levels of 0.25ppm on an API master test kit for example that would equate to around 0.6ppm nitrite. After 2 days the accumulation would already be over 1ppm nitrite early in the cycle but my fish never shown any distress.
I just find this interesting. It's difficult to know if nitrite accumulation has caused a fishes death because you would need ammonia which would most likely do the damage first.
Here the link http://ciresweb.colorado.edu/limnolo...dfs/Pub079.pdf
Feel free to blow my thoughts out of the water or add anything else.