It is generally known as the bio-filter (bio = biological). Some aquarists prefer to name it ‘the life support system’ which is an apt description. Bio-filtration can be provided in more than one way though it all operates on the same principle. In addition the aquarist will be working with Mother Nature as it is bacteria that are relied upon.
Though it is commonly called bio-filtration as said, the bacterial activity that is occurring within the filter is nitrification and denitrification, under the overall heading ‘The Nitrogen Cycle’.
When livestock go through their life functions from day to day ammonia is produced, and ammonia is a deadly toxin. Fish produce the most. In addition rotting food leftovers and dead algae break down and produce more toxins. If there weren’t any bacteria then the fish would start to act strangely, swimming erratically and breathing heavily. Finally they would die, poisoned by the ammonia, so it is clear that the bio-filtration must be present and adequate.
One relevant point that can be made is that an efficient protein skimmer should be used. This is because the skimmer will remove dissolved organics from the seawater before the bio-filter needs to start work on it, thus reducing the work of the bacteria. A protein skimmer does not remove the need for a bio-filter.
How does The Nitrogen Cycle work? Toxic ammonia is the first problem, and bacteria (Nitrosomonas) are present in the bio-filter to deal with this. The ammonia is converted to nitrite.
Unfortunately nitrite is also a toxin, nearly as bad as ammonia, so bacteria (Nitrobacter) are again present to deal with it. The nitrite is converted into nitrate which is not considered toxic, though it is detrimental to livestock at a high level.
It is important that the seawater contains a high level of oxygen as the bacteria converting ammonia and nitrite are oxygen hungry. The term ‘nitrification’ covers the conversion of ammonia and nitrite.
The Nitrogen Cycle ends once the nitrate has been converted by bacteria into gas which escapes from the aquarium at air/water interfaces. This breakdown of nitrate is termed denitrification. The bacteria, in order to break down the nitrate, need a very low oxygen environment. This is because if oxygen were present the bacteria would use it and nitrate would not be reduced. Without an adequate oxygen presence the bacteria extract their oxygen needs from the nitrate, thus breaking it down.
So overall The Nitrogen Cycle is the conversion of ammonia to nitrite, and nitrite to nitrate. Then follows the conversion of nitrate to gas.
Giving the bacteria a home in the aquarium so that they can efficiently carry out their work is easy. First, the design of the aquarium should allow for optimum gas exchange which will permit high oxygen levels in the seawater. Next the media for the bacteria can be considered.
Nowadays the number one recommendation for bio-filtration is live rock. This is so called because the rock naturally harbours the needed bacteria, all of them, those that convert ammonia, nitrite and nitrate. The first two are near or at the surface of the rock, and the third deeper inside where oxygen is not plentiful. It is an excellent medium and in addition provides the aquarist with a natural aquascaping material for a captive reef or in a fish only system. Rock quality must be good and there must be sufficient to meet the demand of the livestock. Live rock is expensive because of the weight which is unfortunate, but nevertheless it should be a first consideration.
If live rock is not used, there are other methods. Probably the easiest is the canister filter, as a huge area of filter media can be provided and setting it up is straightforward. It is important to ensure that the amount of media can cope with the livestock present, so reference to the manufacturer’s recommendations should be made. The media when obtained will be ‘dead’, without any bacteria present. Bacteria are easily introduced by using one of the commercially produced ‘inoculation’ fluids which are easily obtainable. The information supplied should be carefully followed and tests made as instructed.
There is a shortfall with canister filters and similar devices in that the seawater being pumped through the media is oxygen rich. This is excellent for the bacteria that convert ammonia and nitrite, but nitrate will not be touched. Therefore there will be a slow build-up of nitrate in the seawater. I used the word ‘shortfall’ not ‘problem’ as it is easily dealt with. If the aquarist carries out routine seawater changes then the nitrate presence will be continually diluted and should be kept down to reasonable levels. 10% of the system net gallonage is the guideline for initial changes but if necessary this can be increased up to around a limit of 25%. If this is not sufficient to control nitrate then there is something amiss – the system is overloaded with livestock, or the aquarist is overfeeding and the like. There are ways to reduce nitrate by filtration using bacteria (the denitrator filter) but this will not be gone into here. There are other ways too but again they will not be gone into here.
The aim of every aquarist is to own a successful aquarium. There is equipment available to make this more possible. However, at the very heart of the system are the bacteria, essential to every system of whatever type, big or small.