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Old 03-29-2005, 01:14 AM   #1
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Filtration Effectiveness: Alternatives to Gallons per Hour

Measuring a filter's flow in terms of gallons per hour gives a rough understanding of how much filtration it's capable of. However, a simple thought experiment shows that it has its deficiences.

Imagine two nearly identical filters. One uses only a single thickness of cheesecloth as its media. The other uses a foam block, activated carbon, and a Bio-Wheel.

Both will move approximately the same amount of water, but the latter filter will do more filtration work.

Are there other measures of filtration effectiveness that better than gallons per hour?
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Old 03-29-2005, 01:44 AM   #2
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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.
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Old 03-29-2005, 01:48 AM   #3
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I hardly pay attention to gallons per hour except in choosing powerheads or pumps. For filtration I always buy brands that advertise the tank size that the filter can handle. Right now I'm using the Marineland Magnum Bio Pro. The cannister is rated at 350 GPH, but I paid more attention to the fact that each of the 2 Bio Wheels it comes with are rated for 30 gallon tanks, so the filter is basically good for up to 60 gallons (being used on a 39 gallon tank)... Then again... I always try to select a filter that is rated to handle 25-50% higher than my tank due to their numbers usually being based on the absolute limits of the filter rather than the practical use.

I never heard of any hard way to rate a filter's effectiveness except by the manufacturer's claims. As far as the GPH goes... I believe most people recommend using a GPH that will completely circulate the entire tank at least 3 times per hour, so for example: on a 55 gallon you'd want at least 165 GPH, but as above you should try to pick a higher number to be sure you are getting enough circulation.
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Old 03-29-2005, 02:00 AM   #4
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Quote:
Originally Posted by jgc8fan
For filtration I always buy brands that advertise the tank size that the filter can handle...I paid more attention to the fact that each of the 2 Bio Wheels it comes with are rated for 30 gallon tanks, so the filter is basically good for up to 60 gallons (being used on a 39 gallon tank)...
There must be something that filter manufacturers use other than gph when they rate and design their filters. I wouldn't imagine that they just slap together big, medium, and small filters and then figure out what they can handle. I suspect they design the filters for a specific range of tank sizes based on some other measure of effectiveness.

If only they'd share with us...
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Old 03-29-2005, 02:55 AM   #5
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Quote:
Originally Posted by mobiusnu
There must be something that filter manufacturers use other than gph when they rate and design their filters. I wouldn't imagine that they just slap together big, medium, and small filters and then figure out what they can handle. I suspect they design the filters for a specific range of tank sizes based on some other measure of effectiveness.

If only they'd share with us...
The only way I know that they base their rating on would be the effective area of the bio-filter media (ie: the Bio-Wheels advertise 9 miles of surface area per wheel). The area tells how much space the bacteria have to colonize, but very few mfrs even give out that info.
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Old 03-29-2005, 04:28 AM   #6
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Media makes such a difference...like some of the Eheim classic canisters with 8-12L worth of media capacity, which is massive, or the huge media capacity of an AC500 (which can be added to by adding several cubic inches of biomedia around the intake tube chamber).
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