You do not have to use a substrate for a pet turtle, but if you do, gravel is a suitable option. Gravel substrates are aesthetically pleasing, but they tend to trap waste and debris, which slowly pollute tank water. Excessive amounts of gravel exacerbate this problem. Captive turtles may eat small gravel, so the best option is to provide a 1- to 3-inch layer of large, smooth stones in the bottom of the tank.
Aesthetics May Improve
Most basking turtles, such as red-eared sliders (Trachemys scripta elegans) and painted turtles (Chrysemys picta), do not require substrate -- a bare tank bottom suits them well. Gravel mainly serves the keeper, not the turtle, by improving the aesthetics of the tank and anchoring or hiding cage props and tubes. Without a substrate such as gravel trapping debris, a turtle's water stays much cleaner and is easier to maintain. If you can keep their water clean, gravel is an acceptable substrate for most turtles.
Types of Rocks and Gravel
Use large stones rather than small. Large gravel will not trap as much debris as smaller rocks will, which will help keep the water cleaner. Additionally, turtles are less likely to ingest large gravel than small, which can cause life-threatening injuries. Choose smooth river stones that are larger than your turtle’s head, and he will not be able to eat them. Large, smooth stones are also less likely to damage your turtle’s plastron when he dives in the water. Some turtles -- for example soft-shelled turtles -- prefer sand to gravel, which may abrade their shells.
Aquatic turtles are messy animals; keeping their tanks pristine is a tough job in the best of circumstances. Do not make things harder on yourself by using too much gravel. Having more than 3 inches will cause problems. To determine how much you should purchase, multiply the capacity of your tank in gallons by the number of inches of depth you want, which yields the approximate number of pounds you need. For 2-inch deep layer of gravel, a 20-gallon tank requires 40 pounds of gravel. Because of the variation in particle size, this is only an approximation; gravel of smaller dimension would require more than gravel of larger dimension.
Cleaning the Gravel
Periodically, place your turtles in a temporary holding tank and thoroughly clean the gravel. The easiest way to do it is by sweeping the gravel with a siphon-style aquarium vacuum. Proceed systematically, beginning in one corner and working your way around the tank. Be sure to press the siphon deep into the gravel to suck up the polluted water. You can clean sand similarly, but this is a much more difficult task, and you must take care not to stir up dust, which can harm the tank filter.