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Sizing of Filters

Filtration Rate
The generally accepted filtration rate of three four times an hour is not a bad rule of thumb. However, While this isn't a bad rule, it doesn't really tell the whole story.  If we have a 50l tank, and a 200 LPH filter, we really aren't filtering the water four times an hour. Because the water coming out of the filter mixes with unfiltered water, so four times water filtration per will never happen.

Say you have a 100 litre tank with a filter that pumps 20 litres per minute.  The first minute, 20 Litres is filtered, and the remaining water in the tank has 80 unfiltered litres.  The second minute, of the 20 litres that you intake, only 16 litres have not yet been filtered.  Because of mixing, you re-filter four litres.  As time passes, more and more of the water you take in has already been filtered, so you never achieve 100% filtration.  After 31 minutes, only 0.0248 litres of intake have been unfiltered, therefore 99.901% of total volume that has passed through the filter.

99.9% filtration time = 6.2 x Tank Volume / Filter Flow Rate

In the above example:   99.9% filtration time  = (6.2 x 100)/20   = 31 minutes

where filtration time is in minutes,
Tank Volume is in Litres, and,
Filter Flow Rate is in Litres per Minute

The following table shows the coefficient to use in achieving a desired filtration percentage:

Filtration Percentage














The three types of filtration

Mechanical media out suspended solids in the water, is difficult to size mathematically because it varies with the number of fish, the amount of feeding, and how often we want to service the filter. Other factors also determine the amount of media required. Errors in sizing will simply require more frequent maintenance, and will rarely impact water clarity unless maintenance is not performed when required.

When determining the size of the mechanical filter keep in mind the following.

Smaller pore size of the media (media clogs more quickly)

Higher load of suspended solids (more matter available to clog the media)

Smaller surface area of the media (less surface area doing more work)

Smaller pore size (captures more particles)

Greater media depth (provides more opportunities for capture)

Lower flow velocities (prevents dislodgement of captured particles)

Lower pressure (prevents dislodgement)

We have to be somewhat cautious about our design of the mechanical filter, however, because as we increase the ability of the media to capture particles we restrict water flow through the media and increase our system head losses.  The greatest effect we can have on reducing the head losses of our mechanical filter are as follows:

  • increasing surface area
  • increasing media permeability
  • decreasing media depth

Biological media is simply a host for nitrifying bacteia.  The more surface area the more efficient our biological filter will be.  The factors important for our design are:

  • The load of nitrogenous wastes
  • The surface area of the media
  • The flow rate of water through the media
  • The availablility of oxygen in the media

Chemical filtration, when employed, generally requires the use of granular activated carbon or ion exchange resins, such as zeolite clay.  

Staging or order of filtration

The order of the media is very important.

Generally Evalife do not recommend the use of chemical filtration.

That is to say we do not use it. Chemical filtration is used to remove medecines or other substances from time to time.

A system that employs chemical filtration will invariably have a weak bio system. This is because the chemical filtration will remove the by-products that the bio system needs for their existence. If you must use chemical filtration it must be the last stage in your filter system. This allows the bio system every chance to grow to the size required to process all the by-products in the aquarium. If the chemical component is last it will catch any residual left over by the bio-system.

Most canister type filters are pre loaded incorrectly and should be rearranged to put the carbon  etc last.

In any filter system:

the order of filtration should be as follows

  1. Mechanical pre filters
  2. Mechanical medium filters
  3. Mechanical fine filters
  4. Biological filters
  5. Chemical filters