Praziquantel for fluke
Praziquantel is highly effective anthelmintic for the treatment of flukes and worms on fish. (and internal worms in humans, pets and other animals)
Dosage is 2g/1000liter.
First dissolve in 90-95% alcohol, then add water and agitate vigorously before application. (for example shaking a 2-liter bottle)
Fluke will start dying within 3 hours. So you have to make them drunk first!
Although the molar weight is 314 g/mol, the substance is a flake power that is very light. A sizable pond of just 50,000 litre will need a 100g which will be surprising voluminous. Be prepared and for a healthy bill.
Praziquantel is the preferred treatment for flukes on koi because of it's effectiveness (one shot), but it's price makes treating large ponds prohibitively expensive. Most koi shops (used to) stock it - and could be cheaper than Drontic or Drontal, but the 34mg tablet may be more handy for you guys with small tanks. (But watch out for these with combination drugs)