It is very simple to quiet the outflow of a waterfall-type filter. Buy some aquarium sealant (from a pet store), & a sheet of that needle-point sheeting from a craft store. *Remove the filter from tank, and be sure to dry the outflow area very well before gluing the sheeting. 1. Cut the needlepoint sheeting so it fits the width of the outflow area. Please note: If you have the water level down a few inches from the top of the tank, make sure the length of the sheeting is long enough to reach the surface of the water. This is the key to quieting the water outflow. 2. Glue the sheeting with the aquarium sealant to the filter. Use a q-tip (Getting the adhesive on your fingers is very sticky) to press the sheeting against the filter to make contact. 3. Let it dry overnight, and put your filter back on your tank in the morning.  It is amazing how quiet the outflow becomes, and there is practically no disturbance to the frogs at all. The pic below shows what it looks like once complete.