When i was planning my 125 paludarium, i did a ridiculous amount of research because i was going to put dendrobates leucomelas in it. I could not find one documented case of a drowning frog. I decided to put white's tree frogs in it only due to the fact that the plants matched their habitat better. And my frogs LOVE the water!!! I have heard similar from people about darts. In fact i remember one thread where the guy's d. Azureus would actually swim from one side of the tank to the other, under water. I trust 40 million years of evolution. They come from the water as tads, you would think that that trait would be carried onto the morphed frog. The only case i could find of a frog drowning was a guy who "claimed" one of his females sat on another female and held her under water and drowned her as a territorial dispute. He never actually saw it, just found the frog in the shallow water. Is it possible to drown a frog? Most likely, but it's like drowning a dolphin, it's possible, but it ain't easy.......lol i would imagine it would take quite some time to drown a frog. Just leave plenty of areas for them to exit the water and they should be fine.