dones20,
I've never seen ANY toxicity using praziquantel in fishes at 0.20 ppm, and I've performed hundreds of treatments on thousands of fish. The gasping you are seeing is due to the solvent used to get praziquantel to disperse in the water. Prazi is barely soluble in water, so the liquid preparations need to have some other solvent in them. Some public aquariums have tried using ethanol, but this can cause low dissolved oxygen levels, with gasping.
I use praziquantel dispersed into the water through a fine nylon mesh (o.k, I admit it, I use pantyhose for this!).
Jay