It would depend on the type of turtle you had. The smoothie softshells get 14 inches for females and males about half that size. Florida softshell females get 20 inches, Males 12 inches. The Eastern Spiny get around 18 inches.
Here is a picture on a Fla. Softshell.
You measure your turtle from the bottom of its shell. Front to back. That will tell you how many inches it is. For every inch of turtle, you need 10 gallons of tank.
If you had a 5 inch turtle and kept it in a 10 gallon tank, it would be like sticking a 5 inch fish in a 3 gallon container.
If you llove your fish, you are taking a big chance of the turtle eating the fish as it is one of their favorite foods. All it would take is just one moment and your fish is gone. Snails are also part of the turtles diet so if you are swamped with snails, feed to your turtle. If you need to have a bigger tank, one way for now until you save up for one is use a Tote. They have them in 50 gallons at K-Mart for under $15 dollars. Here is one set up.
One thing about turtles are they are messier than fish. You need a filter 3 times the size of your tank. Dirty water means health problems down the road, just like fish.
One thing about turning your turtle lose in the wild after they are all grown up. It could carry diseases that the turtles in the wild don't have. You could wipe out the turtles there where you turned it lose. Post on turtle forums that you are giving your turtle away, look for turtle rescues. But don't just turn it lose.