Well, I'm sorry for your losses. Pesticide or no, nitrite is harmful to fish in any amount. Check your tap water & tank for it. Nitrite can cause "brown blood disease", not instantly fatal but slow. I think it keeps oxygen from being carried well in the blood but don't really remember. Your beneficial bacteria may have been damaged somehow, in a cycled tank there should not ever be any nitrite.
In our old MA house our water came from deep wells in winter (city water supply) & was quite different from summer. Many things like ag run off & "micro-bubbles" can effect water quality & fish health. Sorry, I'm a casual water tester. I know I should do it more often than I do...Do as I say not as I do...at least until you nitrite gets to 0 & stays there...then maybe you can coast...
In our old MA house our water came from deep wells in winter (city water supply) & was quite different from summer. Many things like ag run off & "micro-bubbles" can effect water quality & fish health. Sorry, I'm a casual water tester. I know I should do it more often than I do...Do as I say not as I do...at least until you nitrite gets to 0 & stays there...then maybe you can coast...
