Hi,this topic comes up regularly with Discus keepers,in therory you should only ever need to worm them once or twice,but some people do it every month which I think is pointless unless new fish are being added to the tank unquarantined,the drug used is Flubenol and is now supplied in a 5% strength powder,it used to be 15% so there is some arguement to its efficiancy but I have used it in triple dose when some new fish had white jellyish faeces a sign of worms and it worked,I have read reports that at 5% it is no good,it is supposed to kill snails so I tried it at 5% and they lived at 15% they died,I would worm them in the first instance and as Rob has said Mark at Devotedly Discus sells it,he does not charge p&p so is the cheapest around hope this helps and sorry for prattling on.