Bio-filters in marine aquariums are of enormous importance. Without the nitrogen cycle that is provided by Nature all aquarists would soon be in trouble. All those bacteria working away so industriously are doing us all a favour.
Bio-filtration, or the life support system as some aquarists call it, comes in several guises. The current best is live rock, whether that is aqua cultured or from the ocean. The rock from the ocean offers the possibility of a greater diversity of life in addition to the desired bacteria, but this includes of course unwanted life, such as certain crabs and shrimps, not to mention aiptasia anemones (now aren’t they just every aquarist’s favourites!).
Then there is the canister filter, a device that has been in use for many, many years. They are much improved nowadays with much easier access, meaning that most have got rid of those fiddly little clips that used to be used to keep the lid on. Most come with removable chambers thus making the insertion of media for mechanical, bio or chemical filtration easy. The canister is really well proven and highly reliable. Though not often thought of this way, they really operate a ‘closed loop’ when connected.
There is a further piece of equipment that isflui designed for bio-filtration, but it is not so well known. This is the fluidized bio-filter. Basically the device is a tube usually constructed of acrylic, with an inlet at the bottom and an outlet at the top. Inside is a quantity of sand – not just any sand but a type that resists wear in the tumble action it has to endure. A pump or powerhead is connected to the bottom inlet and the outlet at the top is fed back to the aquarium, usually at the opposite end. Sometimes the device is supplied with an inlet filter to help keep the sand clean, but if not it is easy enough to put a foam filter on the pump or powerhead.
As the seawater is pumped through the sand, the sand is ‘fluidized’. This means that the flow of seawater pushes the sand upwards with enough force to keep it loose, but not enough force to make it hurl about the tube. The result is that each grain of sand presents all of its area for colonization by bacteria. Such is the area that a fairly small unit can deal with a fairly large aquarium. Of course this also depends on the bio-load – a fish only system usually presents a larger load than a reef system.
The downside of this type of bio-filtration is that the nitrogen cycle will operate very efficiently, the bacteria converting ammonia to nitrite, then nitrite to nitrate, but it stops there. So nitrate will slowly build up in the seawater degrading its quality. This also applies to a canister filter.
The fluidized bio-filter is a perfectly viable alternative when a system is being considered. The units are usually ‘hang-on’ and easily fitted. The sand in the tube tends to degrade over a long period and the manufacturer often puts in a small bag for replacement purposes. The manufacturer will also advise the flow rate that is required to fluidize the sand correctly.
It is important that the aquarist maintains good general seawater circulation in the display aquarium as the bacteria that inhabit this type of device depend on the seawater’s oxygen content, in the same way as a canister filter and live rock. Also, in addition to the other benefits, routine seawater changes are required to control any nitrate build up.