Good point, mobiusnu.
The manufacturers' GPH
ratings are a bit misleading because they are based on the filter's ability to move water in the absence of any filter media. A cannister filter rated at 500 GPH
is NOT turning over 500 GPH
when the media baskets are full of ceramic noodles, floss, peat, or whatever. The cheesecloth filter in your example would probably put out 500 GPH
, while the foam/charcoal/biowheel filter might only put out 300 GPH
. But as you have brought up, the point of a filter is not simply to move water; it's to filter the water.
I guess the best measure of a filter's effectiveness is your ammonia, nitrite, and nitrate readings. Of course, those also depend upon how often you do water changes and gravel vacs. Other useful measures might include water turbity (absorbance) and dissolved solids (conductivity), although few aquarists possess the equipment necessary to take these readings.
The rule of thumb is 5 - 10 times tank volume per hour - more if you have messy fish like goldies or oscars. Most people here would agree that you cannot overfilter, unless your fish are being thrown across the tank by the current.