I would guess that some of you are using a venturi on your powerheads to get oxygen into the water, but this will result in boubles in your tank, so what you do it direct some one of the powerheads so that they break the surface of the water, and the oxygen will get into the water in this mannor. It dosn't have to be much, as long as the water ripples, then it will allow for carbon dioxide to leave and oxygen to come in. It is like this waves oxygenate the water.
