I’m assuming we’re talking of bio filtration. In any case, the coarse and fine media would be the same whether the canister was used for bio or purely mechanical purposes.

The normal method of filtering mechanically in a canister is to use medium (sometimes calles ‘coarse’) and fine sponge pads. The medium pad(s) comes first followed by the fine. Some aquarists use a very coarse particle media as the first filtration bed, but personally I have found them unnecessary.

If the filtration is purely mechanical, then the canister can be filled with many sponge filter pads and also, if required, the last step can be another filtration media such as carbon.

If bio-filtration is required, what I did was place all of the mechanical filtration (ie sponge pads, medium and fine) before the bio-media. In this way the bio-media was generally clean even after 6 years of use. Of course, it is necessary to be sure there is enough bio-media to filter the load in the aquarium.

Particularly with a bio-filter, it is very important to regularly clean the sponge filter pads. It has been stated that they should be cleaned as soon as there is a reduction in seawater flow. In my opinion, this is not good enough. Cleaning should be regular enough to ensure the continuing proper water flow.

Cleaning the sponge pads can be done under tap water, squeezing them out until clean. Under no circumstances should a sponge pad be cleaned under tap water if that sponge is the bio-media, this would be disastrous.

There are other ways of mechanical filtering other than sponge but generally this is the most convenient and easy to maintain media.
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