maybe you are treating for the wrong thing mate? i have used metro a few times before, at quite high dosage, and its never done any harm to fish, or anything else.
for medicated food just use flake, mix with water into a paste and add powdered metro. then flatten out between gladwrap and dry out again. not sure on the dosage, but it has been posted on here not so long ago.
this is the method i have used and its worked for me.
good luck
just a thought, have u tested your water?