TREATING SICK FISH IN REEF TANK?

i have a dsb and live rock. i was out of the fish tank hobby for 8 yrs just got back in. i hve two tanks both are doing well .. however i spoke to my lfs about fish getting sick and having lr in tank and i got scared. i forget the disease he spoke about that he said its in most fish but comes out evenutally in all .. anyhow u use copper to treat and he said if you hve live rock you will kill the organisms on the rock and pollute your tank possibly causing a recycle killin off all of your stock? is this true? how often if ever do fish need to be treated with copper . what r the risks of using in live rock dsb tank with no inverts or corals? are there alternatives? thank you so much for any help
 
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