Not sure on how it relates to Prazi (LHG is your man there) but I wouldn't get in to the habit of using medications in a prophylactic (precautionary) manner. All medications are toxic to fish to differing degrees, you want to limit exposure to them as much as possible. Wait until you have symptoms and can correctly diagnose a condition before medicating.
Drontal often contains both pyrantel embolate which kills roundworm and tapes etc, and some formulations contain praziquantel too, which seems to work well on flukes, and a few acarids . Drontal is a trade name though, a brand and the formulation may vary depending on the animal its intended for use on. Droncit is the praziquantel only equivalent, and is probably more practical and safer for use with fish. Doses depend on listed concentrate of active ingredient, and also has to be balanced against method of solubility and intake method into the fish, and the relative size and weight of the fish, be it water administered or added into a food. The dose will also vary according to the parasite type your trying to eliminate. Tapeworm, camallanus and blood flukes may require double dosages depending on exact formula, and there are some parasites that prazi cant shift. Length of treatment times may vary too.
Ah thanks for that. I was just thinking of using it on any new fish I get, during quarantine to eliminate any possible parasites before introducing to my own fish. Sounds like it would be impossible to calculate a dosage, though!
If you do eventually need a treatment with praziquantel in it then why not just buy some Sera Med Professional Tremazol. Any LFS who stocks the Sera range should have it in stock or you can get it on ebay, or just google Tremazol. HTH