About Using Canister Filters

Tropical fish aquarium

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Used for mechanical, chemical, and/or biological filtration in saltwater aquarium systems, canister filters are quite versatile. A canister filter can be used continuously or added and run only when needed. They can act as a stand-alone filter, or be used in combination with other types of filtration as well. Here are some examples for each category.

For Mechanical Filtration 

An aquarist with an under gravel filter can add a hang-on-tank canister filter to their system to remove free-floating particulate matter from the water that would normally get drawn down into and trapped in the substrate. By continually running a canister filter on this type of aquarium set up, it contributes to improved water quality.

An aquarist that has a semi-reef system with fish and some live rock in it can choose to install a canister filter in line to act as a "pre-filter" to remove unwanted waste, particulates and detritus from their tank water before it passes into or through their biological filter (i.e. wet/dry trickle) or main tank water supply, such as into a sump. This can also be done on full reef tanks with live rock and corals in them, but the debate about continually running mechanical filtration in this type of system is that such a setup filters out beneficial plankton life in the water that many marine organisms feed on.

Hang-on-tank canister filters are very often only used as a means of mechanical filtration during regular tank cleaning and maintenance care routines. They are also one of the simplest ways to control heavy copepod and amphipod larval blooms. You know, those little white bugs often seen swimming or crawling around in your aquarium.

For Chemical Filtration 

For an aquarist that needs some help in clearing up a water quality problem, they can place granular activated carbon (GAC) in the media chamber to help eliminate odors, medications or other contaminants in the water, as well as use other types of absorbing products that are designed to remove nitrates, phosphates, silicates and other unwanted chemical elements or compounds. This type of filtration also applies to the filtering of fresh tap water prior to using it to make-up sea salt mixes or adding it to an aquarium as top-off water.

For Biological Filtration ​

Even though many canister filters are designed for this purpose and a lot of aquarists use them in this way, in our opinion they are not a good choice as a "main" source for biological filtration. They may be o.k. for smaller systems, but most do not have a chamber big enough to hold a sufficient amount of bio-media in them for larger ones. Therefore, they are inadequate to use solely for this reason, but one can still be run in conjunction with another form of biological filters, such as with live rock or a wet/dry trickle filter, for additional mechanical filtration of the aquarium water.

Now, one of the most important things to consider when purchasing a canister filter is the water flow rate, NOT based on what the manufacturer says you will get, but what you will ACTUALLY get after taking a few other factors into consideration.

Water Flow Rate Considerations 

The flow rate on the manufacturer's instructions is 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 affect or diminish the flow rate of a canister filter.

  1. Using extra media (carbon, etc.) or filter materials (poly filters or floss, micro pleat cartridges, sponges, etc.) inside the media chamber of the canister.
  2. Canister filters are often stored underneath an aquarium, so there is a lot of head pressure to deal with, and possibly a long distance 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.

Taking the above two factors into consideration, the exact water flow rate can be determined after you buy a canister filter and have it up and running on your aquarium, or you can get an estimate by applying the flow rate equation in Step 7 of our How to Determine GPH Water Flow Ratesbefore you buy one. If after getting your estimate, taking the other factors into consideration, you are still not sure if your choice is a good one or not, it doesn't hurt to purchase a larger filter than you think you will need. You can never turn your tank water over too many times, but you can have a problem with not turning the water over enough because this can result in poor water quality. A rate of 6-10 times per hour of tank water turnover time is recommended.

Other Feature Considerations 

There are many brands of canister filters on the market to choose from. Some of the most popular traditional canisters are EheimFluval, 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.

Here are some maintenance tips that will help prevent or lessen the further diminishing of a canister filter's water flow rate and operating efficiency.

  • Place your canister filter in an easy to get to spot for cleaning. If you don't, maintenance will be neglected because it's a hassle to service the unit.
  • If the manufacturer of the canister filter you buy offers any useful tools for making the task of cleaning the unit a faster, easier, and drier one, in all likelihood the more often you will do it! This we learned first hand after adding the double quick-disconnect valves to our Marineland Magnum 350 canister filter and purchased the handy attachable Power Kleen Gravel Washer to periodically siphon clean the bottom of our aquarium.
  • Purchase extra micron-pleat cartridges, floss, sponges, granular activated carbon (GAC), and other filtering materials or absorbing products (read reviews & compare prices) to be used in the canister. By doing so you can quickly change or rotate the material in minutes, eliminating the hassle of having to remove and clean or rejuvenate the only medium you have on hand before you can place it back into the filter.
  • You will notice a slow down in your water flow rate when the filtration material in the canister becomes dirty. You can never clean or change it too often unless of course, you are using the canister for biological filtration purposes. If not, the filtering media or material should be cleaned or changed regularly, at least once a week. If left dirty this will decrease your water flow rate, as well as allow for the accumulation of an unwanted waste load on your system, which in turn contributes to poor water quality problems, the number one being an excess build-up of nitrates the aquarium.
  • Because algae, calcium, and salt crystal build-up can occur inside the hoses, canister chamber, impeller and other areas of the unit where water passes through, at least once a month the filter should be taken completely apart and all sections cleaned to remove any possible blockages.
  • Don't be concerned about losing any biological bacteria from cleaning or changing your filter material. Remember, the purpose of a basic canister filter is to remove and clean your tank water of waste and debris. It's not a biological filter, unless of course for some reason you are using it for that purpose.