'Sand Filter'

Although not really a filter in the conventional meaning of the word sand can play a vital role in water quality. This post in an extension and revision of previous posts that tentatively explained the importance that I gave to live sand as a filter in the aquarium. I feel I am closer to explaining why I feel sand is so useful. Hopefully this post will clarify.

Sandy shores are very different to rocky shores in basic ecology. There is no primary producer on a sandy shore except perhaps for seaonal diatom blooms on more stable surfaces or eelgrass beds. Food supplies are either plankton available for filter feeders such as cockles, or detritus. The fauna can be roughly split into three categories according to size.

1. Macrofauna - Cockles, lugworms, crabs etc. Animals over 1mm.
2. Meiofauna - animals smaller than 1mm larger than 0.05mm
3. Microfauna - animals smaller than 0.05mm

Almost all of these animals are infaunal - they live within the sand. The vast majority are detritivores. Within the meiofauna alone there are 22 known Phyla! The amount of biomass is heavily dependant upon the particle size of the sand or mud. The finer the particle the greater the number of animals. Coarse grained sands are practical deserts, highly oxygenated but relatively devoid of life. Very fine sands are teeming with life even though only the top few centimetres are oxygenated and are often anaerobic below 80mm or so.

It follows that very fine sands, rich in meiofauna and microfauna can be effective at consuming uneaten food, fish and mollusc waste and decaying plant matter. This consumption appears to have a net reduction effect upon the nitrate levels as opposed to more conventional mechanical filtration heavily geared towards mineralisation (the conversion of organic compounds to inorganic).

My own aquarium, with a 120mm layer of very fine 'live' sand has, so far, maintained high water quality despite no cycling. heavy stocking and frequent feeding. Its my belief that the sand has played a vital role in maintaining ammonia, nitrite and nitrate at zero. Obviously a raft of other factors have also played important roles. Lighting to allow algal growth that takes up any nutrients and minerals, high flow within the display tank to keep things moving, heavy and constant skimming to remove soluble proteins, chilling to provide the correct temperature range for native organisms to thrive and planted live rocks encrusted with different animals and plants are all undoubtably important.