I have a fishkeeping mistake to add to this forum, haha. I've had a lot of mistakes but this one was by far the worst.
Years and years ago I had a beautiful amazon-themed 10 gallon community tank. It had heaps of fish in it and was looking really nice.
anyway I had a shoal of about six rummy-nosed tetras and they got ich. I think they had it in the shop and it only appeared later. I got the treatment. it said you had to remove the carbon from the filter. at the time I had this really painful canister filter and it was easier to just turn the whole filter off for the two weeks of treatment than disassemble the whole thing to remove the carbon. (if only I'd known..haha)
well after two weeks the tetras were better and I went to turn the filter on again.
It had been sitting there stagnant for two weeks.
within a few hours all the fish were gasping for breath at the surface, or rolling on their sides, except for the bristlenose catfish which simply couldn't suck onto anything anymore...they just sort of drifted around the bottom.
in the two weeks the filter was turned off it had grown some sort of highly toxic bacteria or something. possibly anaerobic.
I lost almost an entire tankful of fish. I think the only ones that survived were my peppered catfish, which have been through thick and thin and died only recently.
I'm never doing that again.