How much iron are you adding?? <Calculate the ppm
of iron added to your tank: (content of Fe
in fertilizer in mg
/ml x amount added in ml)/amount of water in tank in liters>
People add 0.1-0.3 ppm
weekly without problem, and I am sure higher doses are OK for fish. Inorganic iron is not that well absorbed, and most iron preps are chelated anyway. <Chelation agent is what you use to get rid of iron from your body in case of posioning.>
I don't know about fish, but in people, Fe
posioning causes liver problems & you really need hugh amounts given parentally <ie not by mouth>. I very much doubt your tank has that much iron to cause fish death.
Now, most people dose Fe
as trace mixes, which has other stuff .... overdose of these (copper comes to mind) might give you problems.