It is possible these are Salt Marsh Caterpillars. They do feed on lily pads and are common in TX. If you do a google on them you can find all kinds of information.

Bacillus thuringiensis is recommended for caterpillars and doesn't harm birds or other insects as it is a naturally occuring bacteria that kills only grubs and caterpillars. However I don't know if it has any negative effect on fish. You might have to hand pick the ones you can reach. Hopefully someone else can jump in with better info than I have.

I don't see how bringing in fish could introduce these to your pond unless you also brought in plant material that might have had eggs on it. Maybe it is just a coincidence.

Good luck.
Anne

Thank you, Anne. I went out this afternoon and killed as many as I could reach. I used water pressure from my hose to knock off any remaining ones. I hope they drown or are eaten by my goldfish.

Anne, BT has no affect on fish, frogs, snails, or any other aquatic creature.