you cannot treat the flu if you don't have it.. not to say you won't get the flu down the road.. so treating oscars that do not have it yet will be no help... just improve their conditions the best you can.. treat the one who has it in a separate tank and hope for the best.. they are all very susceptible to this.zebvance;4532833; said:Entire tank. If one has it almost def means that the others will get it or have it and are just not showing signs yet. It is said to be caused from high nitrates and stress from tanks being to small.