Depends on the brand..Aqua-cure Tetracycline is used at the rate of 4 g (one level teaspoon) per 20 litres. It will definately wipe out your bio filter, so be prepared.
Thirty-five years keeping and breeding discus, and I'm still learning :P
Using Tetracycline,
Say your treating for Discus disease with Tetracycline in your hospital tank, does your main display tank have to be treated if discus desease was first diagnosed in that tank(dispaly)