I learned a lesson the hard way today. That lesson: make sure there are no openings in your tank top. I came home to find an empty 5 gallon tank where Ty, our betta should have been. I looked on the floor behind the stand and there he was. The only opening is a 1.5" hole where the heater goes in. He apparently jumped through that fairly soon after we fed him and left for work since he was completely dried out. Go check your tanks now and cover even the smallest hole so you won't come home to what I did.