7 inch turtle live in a 20 gallon tank?

Depending upon the species of turtle you wish to house, you could be OK or run into trouble. The tank may also need to be set up as a Vivarium with both wet and dry areas and possibly a heat lamp for the turtle to bask under. There is much more to keeping Amphibians than simply sticking one in a tank full of water. BTW. NEVER put Amphibians with fish.