I actually noticed that some Algae Wafers have some copper sulfate in them.. Since that is a large part of their diet, thats what might be hurting them.
The amount in food should be so minimal that it should make no difference. Almost all foods have things that are bad for your fish or animals, but not in the amounts consumed in food. It's like drinking tap water. I'm sure drinking chlorine can't be good for humans, but the amount is so minimal.