Diesase goldfish

Question
I thought that I was doing everything right, then this week I noticed one of my goldfish was lathargic, not eating and sitting at the bottom of the take. He/she also is having problems with their fins, they are slowly dissapearing. I have removed this fish from my 55 gallon tank getting him away from his/her other 5 siblings (all goldfish). I have been treating the sick fish with quick cure or melafix. Hes maybe not quite as lethargic but hes still not eating and just staying at the bottom of the 10 gallon tank. Then this morning I was looking back into my 55 gallon tank and another fish has white circles on his body and fin. What am I doing wrong? I don't want to lose any of the fish I have, but i don't know why I am having so many problems? I clean the fish tank every month taking out 50% of water, I add salt to the water, and I change the filter every month. Could you please let me know how to save the sick one, possible two and make sure that the other goldfish do not get sick. I wanted to have these fish fro a long time hopeing that they get as big as I have heard they get, but I'm starting to get a little discouraged. Any help/advise you can give me would be greatly appreciated.

Answer
the white circles sound like ick, you can get ick treatment at the petstore...

fin dissappearing could be fin rot, you can also get fin rot treatment..

try cleaning out your tank weekly and do 20% water changes rather than 50% monthly as that will shock your fish =]


good luck x