Hi Harry,
Prophylactic treating of your fish on a weekly or even monthly basis isnt needed and not something I would recommend..Its pointless and waste money and exposes your fish to unneeded chemical stress which could damage organs. Its a good idea to deworm your fish once when you first get them and they are in quarantine. If you do this everytime you get new fish in and do not introduce any more worms with new fish or contaminated foods you should not have to retreat for worms.
Levamisol is an excellent choice as a general dewormer Its effective as a 24 hour bath at 2ppm.Most use it as a bath 2-4 ppm and its generally easy on discus at that dose.. 6ppm and up has been reported to stress Discus and above 10 ppm has killed discus by some accounts. The dosing is generally done on day 1, day 7, and day 21 in the hobby. The spacing is not hard science but guesswork meant to cover multiple species of worms life cycle.
You can use levamisole in the food instead
Another effective dewormer is fenbendazole .Only use it in the food. As a bath its toxic to discus.
hth,
Al
ps.. You may find this article interesting.
http://www.simplydiscus.com/library/...ematodes.shtml