Your turtle’s tank should not smell if you’re cleaning it regularly. The most common cause of the odor is the build-up of waste like poop and leftover food. Too much algae in the tank can produce a foul odor as well.  Fortunately, it’s easy to prevent your turtle tank from smelling — just clean it regularly. Spot clean every day to remove leftover food and poop from the water and terrestrial habitat. Make sure you have an excellent filter that sucks the waste from the water. Plus, perform water changes every one to two weeks to eliminate old, smelly water.