i have one of the filters that hangs over the back of the tank. (described it in "got a free tank today" thread. but once i got everyone going..the hole where the "stem" goes into the bottom of the filter, by the pump, leaks a little water, which will be a lot of water say overnight. what would be good to seal it up with? some of that tank sealant? or just some plain old silicone from the hardware store. didn't know if the silicone would be safe for the fish.