Aquatic tiny worm-like parasite?

I have a brackish water (part fresh-, part saltwater) aquarium with 8 spotted freshwater puffer fish in it and 1 African leaf fish. They are fed only frozen bloodworms and small aquarium snails. I live in southwest Florida and use tap water that is treated for the fish. I go to Petsmart sometimes and take some of the snails invading their aquariums to feed my fish and have recently noticed this very tiny hair-like worm parasite that swims up out of the rocks in the aquarium every now and then in a squiggly motion. An employee there stated that they are aware of these "worms" and that they believe they are from a fire-belly toad aquarium but are really unsure as to what they are. I'm having a hard time getting rid of them, though they don't seem to be directly affecting my fish. I know they're probably not anchor worms, because they don't attach themselves to the fish. I can sometimes see them curl up in a ball and when they do this it's hard to keep track of them, because they end up looking like the little air bubbles floating through the water. If anyone has any idea what these are and how to get rid of them, please let me know all that I can do and what to buy. I've only tried these alka-seltzer-looking tablets that you drop in the aquarium (I forget what they are called) that are supposed to get rid of some aquarium parasites and then when that didn't work, I tried simply cleaning out the aquarium and everything in it with scorching hot water, including replacing the filter inserts and everything. I just don't want to use anything dangerous for my puffers. Please help!