i have three diamondback terrapins that i rescued about a year ago. when i found them in the street they were no bigger then a half dollar but ive been keeping them in a freshwater tank. i researched them and it is said that if they stay too long in fresh water they will develop shell infections, but i havent noticed anything wrong with their shells yet and its been almost a year. should i still set up a tank with brackish water for them?