On the bright side, the frogs will take care of any potential mosquito problem, and you should be much more concerned about the health risks surrounding mosquitos than the noise the frogs make. I know, that's easy for me to say, etc., and I don't mean to belittle your problem, I just wanted to warn you that you may not want to take care of the frog problem while inadvertantly exacerbating the mosquito problem. You need to fill the pool. So: dirt. Lots of it. Nothing fancy, but not pure clay neither.