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Will copper sulfate hurt a BN pleco? I need to rid a tank of snails.

Why not first netting the fish and than draining the tank?

If you go messing with chemicals, chanses are you will firstly miscalulate and kill every animal in the tank and secondly the copper will attach itself to the substrate and so on, impying a furhter dosing - with all accompianing risks, among which rendering the stuff unsuitable for further use

Better to set yourself a daily mission of removing lets say manually ~50 snails per day until the epidemic is under control.