Question About Pets ( in tanks)?

Okay Well I have A frog tank with 2 fire belly Toads and a fire belly newt. i keep them in a 10 gallon fish tank. the water is about 4 inches high right now. but in 3 days there will be about 1 and a half inches. i have to refill it about once a week. and I'm sick of it! and there aren't any leaks because everywhere around the tank is dry. why does it keep emptying? i live in New york so its like 30* degrees every day so its not evaporating. and its always at room temperature