You should fill the tank to about an inch from the top, so the frogs have an air pocket, for when they rise to the surface to gulp some air. They need a Tight screen top so air can get in, but they cant fly out. I would use the square 2-gallon tank and change half of the water weekly. You need to use aged tapwater to avoid chlorinated water. You are smart to look for info before you get the frogs. So many people just buy them, and then ask the questions, but it is better to read everything you can before hand!