Maddy- It sounds like your tank size is fine. The temp is exceptable too. Glad to see you have changed your diet. I have no idea what killed you first two frog, but yoy made a sound decision in not buying anymore until you sort this out. It is highly possible your frog is not eating(at least in part) because of all of the recent changes. New diet, new digs, no tank mates. Is the new setup cycled yet?