Praziquantel is a very safe medication. I've used it on my fish twice, as a prophylactic measure, and there were no side effects. I used praziquantel pills that were ground up with a mortar and pestle and dissolved in water. It clouded my tank for ~10 minutes or so, and when the cloud was moving towards my fish they tried to get away.
But once exposed to the medication, they didn't react negatively. I can't say whether or not it worked because as I mentioned, it was a precautionary measure... but it has a very good record with humans and animals as being a safe and extremely efficient deworm medication.
If you use the proper dose, I wouldn't be worried at all about using it with fish or plants.