Is it ok to use gravel in a tank for frogs, or should I use something like soil? The gravel seems to be the only thing that won't float when I put the water in, and it looks nicer. However, for fire bellied toads and plants, is this ok? The guy at the store said that this other stuff was better (I think it was coconut fiber substrate,) but it was floating in the water. The thing I was worried about is if they could swallow the gravel. Would sand and dirt (from the store) be better?