Aquatics Unlimited: Articles: Slow Flow Canister Filters
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Slow Flow Canister Filters

for the week of 3/25/99

Represented by the Eheim® and Fluval® lines, slow flow canister filters provide a reduced, gentler flow of water than their fast flow cousins, and hence do not collect solid waste as effectively.

Advantages. Since they allow solid waste to settle into the aquarium bottom, slow flow filters can go for weeks or even months before clogging. And as the solid waste breaks down into dissolved waste, the slow flow filter can remove it both chemically and biologically, if outfitted with the proper media. The gentle current of the slow flow's output aerates the aquarium less efficiently, but since the flow rate stays steady for longer periods, the aeration is also more consistent

Disadvantages. In crowded or heavily fed aquariums, solid waste may quickly become unattractive, and more frequent water changes may be required. Also, filter carbons and other adsoptive media begin releasing captured waste products after a few weeks, so allowing the filter to go much longer between cleanings may be counterproductive.

Best tanks to use slow flow filters. Slow flow filters can be used on almost any aquarium, but are ideal for tanks containing few or smaller fish with limited solid waste output. They are particularly useful on heavily planted "natural" aquariums, where fast currents can cause damage to plants and also drive out carbon dioxide.

Sizing. A slow flow filter should turn over the volume of the tank somewhere in the neighborhood of 2 to 4 times per hour. Thus, a 30 gallon aquarium filter should be rated at 60 to 120 gallons per hour. Oversizing a slow flow filter (for example, using a filter with a 240 gallon per hour output on a 30 gallon tank) turns it into a fast flow filter, with all its advantages and disadvantages.


"Tip of the week" appeared regularly in 1999 and 2000.

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