Can anyone help me with ANOTHER TRopical Fish question?

since posting last ques.im not sure what to do-My fish had ick prob.from slight temp drop for app a day also saw signs of (thanks to people answering my ques.) poss.Finrot.I lost a12" pleco in a matter of 1 day his fins completely disintegrated as he slowly died:( Left in my 46 gal tank are a 12" goldfish 2 small app 3" fancy goldfish and a 3-4" ghost catfish-looks like askeleton Im just learning that i may be overstocked that just1goldfish req. a 30gal tank& ea.one after an add.10 gal.*My problem IS since treating the ick-only 3 days ago those signs went away quickly its a 40 day treatment & they say doNOT do a water change during that time.The signs of finrot/cloudyeye/both?have remained MAYBE worse & now the cloudyeye-looks like a SCRATCH is not only on my big goldie but also on1of the lil guys.Im goin to the store to get meds for finrot BUT i understand you must elliminate the source-bacteria- by doing at least 1water change which CONFLICTS with the ick treatment WHAT DO I DO?!?!?