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OP said they stayed at the surface and then went to the bottom as they died.IF there was a poisonous atmosphere in the garage you would have noticed it, or not lived to write about the tank issue.
The issue seems to be inside the tank, and toxic to fish. Something causes rapid breathing and quick death. IF they were low on oxygen they probably would go to the tank surface where O2 would be more available. OP said the fish were on the bottom of the tank, with rapid breathing.
The mystery remains
That is just not a safe assumption. Everything is affected differently by CO. Rats are much more sensitive than humans and in general smaller animals can be much more sensitive and fish are very characteristically different than mammals so there is a chance they are even more or less sensative I don't know the answer. I was able to find very little science on the matter. There is an article on the use of CO to anesthetize fish and one of the benefits is as a color preserver and anti-microbial agent which does jive with your statement that the fish 'looked great' after they passed.It’s cool. I would just think if it was CO poisoning that everything else would have died to and not just the fish.
I really don't know what levels are unsafe for fish. I know that CO detectors are generally pretty sensative but for more specific info maybe someone a whole lot smarter than me could help? @Jay Hemdal @vetteguy53081 @ReefSquadAll good man. I appreciate the interest. What you said is intriguing. It’s a natural gas tankless water heater approx 20 ft away from the fish tank. Of course it’s built to code with all the necessary exhaust and there are a few vents in the garage as well. How low of ppm do you think could be deadly to fish? And would a standard store have a monitor that read a trace amount?