I have used under gravel filters since the 60's and have never had problems with my tanks. This past year I have had to set up all my tanks fresh and have tried a little gravel in the bottom with a SPONGE FILTER. I am using a HYDRO II, which is rated for a 40 gl. tank ,in a 20 gl. and 15 gl. tank and they STAY NASTY! How do you use these things? I have tried them with and with out air stones but my tanks are still NASTY!!! Why does no one use under gravel filters anymore?