Do you gyes know this one. I was reading a article and it said to always have the water lower than the top of the tank inorder to allow the fish to breath up there. It really didn't state though how low I need the water to be inorder to provide this. I suppose inorder for the water to continually take in oxygen from the air there needs to be an adequate space inbetween the top of the tank and the water surface, I think. Don't know how much this distance should be though. 
Thanks
Thanks