my fantail goldfish have fungus type lumps

[kendra20]

my fantail goldfish keep developing fungus type lumps on their heads, I have been treating the water with a liquid from the pet shop that is made to get rid of it but it keeps coming back. they are in a clean well filtered tank and are cleaned out once a week my other fish are fine. there are 7 fish in a 2ft xx 2ft tank.