Does a sand fish live under the water?

No, Frogs do not need sand to live in the water. Frogs though may elect to live in swim in sand many are just as happy to bury themselves in small pebbles, mud, or other fine substrate. This practice is to hide from predators such as snakes. If there is no predators, or species that the frog views as a potential predator, a frog will be fine to leave in the water with no substrate at all. Though the frog is safe and can survive this will likely cause the animal stress.

Simply to answer in a line all frogs need to survive is a source of oxygen and a source of food, not sand.