The red-eared slider is a small species of turtle native to the southern United States, now found all over North America as an invasive species. The turtles often are kept as pets, both indoors and outdoors. Many reasons could explain why worms would be located in the water with red-eared sliders.
Worms are one of the red-eared slider's staple foods, both in the wild and captivity. In the wild, they eat earthworms and grubs. In captivity, they can eat earthworms, mealworms, wax worms, silkworms and horn worms. Earthworms can be found outside. The other types of worms can be purchased at most pet stores that also sell reptiles. These worms can be placed directly in the red-eared slider's cage, but should be removed if they sink to the bottom before the turtle eats them. Otherwise, they might decay in the water.
If the water in an outdoor pond that houses turtles is left stagnant for any period of time in the spring, mosquitoes can lay their eggs in the water. When these eggs hatch into mosquito larvae, they resemble tiny black worms swimming in the water. Empty pots and puddles also can harbor mosquito larvae. Homeowners can pour bleach in the water to kill the larvae to prevent a heavy mosquito population on their property. However, in a turtle pond, red-eared sliders will consider the larvae as a tasty snack.
Tiny worms in the water could be parasites from inside of the turtle. Turtles eat and defecate in the same water. If they have internal parasites, the worms will eventually be floating in or swimming in the water. A fecal sample can be taken to a vet to determine what type of parasite or worm the turtle has; the vet will prescribe medication for the turtle. The turtle's tank will have to be scrubbed clean with bleach to kill remaining parasites so your turtle doesn't become reinfected.
Red-eared sliders shed their skin, just like other reptiles. Because red-eared sliders live in water, the shed skin resembles thin, white worms floating in the water. If you are concerned about worms in your red-eared slider's water, examine them closely to ensure they aren't pieces of shed skin.