Water Flow Rate Considerations
The flow rate on the manufacturer's instructions are reflected as an empty filter with zero head pressure (the canister is not having to pump water uphill). There are two basic factors that will effect or diminish the flow rate of a canister filter.
- Using extra media (carbon, etc.) or filter materials (poly filters or floss, micron pleat cartridges, sponges, etc.) inside the media chamber of the canister.
- Canister filters are often stored underneath an aquarium, so there is a lot of head pressure to deal with, and possibly a long distance of hose the water has to travel through. For the "hang-on-tank" type canister filter, you will get a water flow rate closer to what the manufacturer states, because there is little or no head pressure to contend with.
Other Feature Considerations
There are many brands of canister filters on the market to choose from. Some of the most popular traditional canisters are Ehiem, Fluval and Magnum, just to name a few (read reviews & compare prices). No matter what brand you buy, each manufacturer has their own distinctive characteristics in design, so doing your research on the different types is very important. Some are designed with special features, such as ones that use powder or diatomaceous earth to step the filtration up to very fine levels. Robert Fenner says these types can be sub-classified as "pressurized filters" that can really jack up your electric bill, so they are better used periodically rather than continually.
The best way to figure out what kind of canister filter you need is to determine what function you want it to accomplish, then decide from there which kind you should buy.
Another factor that can contribute to a canister filter not running at top efficiency and slow the water flow rate is ignoring proper or regular cleaning of the unit.