Filtration is at the heart of the aquarium system. It is essential for the well being of the livestock.
Modern filtration systems usually involve [tag-self]live rock[/tag-self] perhaps with a deep sand bed (DSB). The trend is towards more natural filtration, and often macro algae such as [tag-self]Caulerpa[/tag-self] is used to assist in mopping up nutrients such as phosphate and nitrate. Filtration is assisted with an efficient [tag-self]protein skimmer[/tag-self].
So what of older filtration methods. I would have thought that under gravel filtration is no longer used in marine systems, though maybe there are the odd one or two. Canister filters are still used, mainly for mechanical and/or chemical filtration, but also on occasion for bio-filtration. Then there is the trickle filter, sometimes called the wet and dry system.
The system is sometimes called wet and dry because half is out of the water and half is submerged. Bio media is placed in a tray or trays above the water level, and a further tray or trays is below the water level and under the upper trays. A pump delivers water to the top of the tray stack, and the water trickles down through all the bio media until it rejoins the aquarium. The water is normally pumped up to the aquarium and overflows down to the trickle filter.
These filters need very little maintenance except the need to regularly rinse or change the mechanical filter at the very top. This filter is very important as it stops the bio media from filling with debris, particularly the upper tray(s).
The great advantage of the trickle filter is that the media in the top half is exposed to air. This means that there is around twenty times more oxygen available to the bacteria than to those under the water. This helps make the bio action of the filter very efficient.
The filter can usually easily be accommodated as a complete unit under the aquarium, or incorporated into a sump. Some aquarists have a trickle filter in a sump alongside macro algae and/or a DSB.
Another advantage of the trickle filter is that the aquarium water is being exposed to the air therefore oxygen uptake occurs. Bacteria in bio filters are oxygen hungry and in most filters this can only come from the aquarium water. In this case there is plenty of available oxygen for the bacteria and the water.
So if this filter is so efficient are there any negatives? Yes, there are. The major one is the same as for canister filters – nitrate. Unlike high quality live rock, the trickle filter cannot deal with nitrate. The level of nitrate production depends on the material within the seawater to produce it and this is true for any filtration method – canister to live rock. However, the full nitrogen cycle cannot be achieved with a trickle filter and other means are required to control the level. This includes the already mentioned DSB and live rock, also macro algae. At the end of consideration is the denitrification filter. This filter forces the bacteria within it to extract oxygen from nitrate because the bacteria live in a very low to no oxygen environment. Of course, the requirement to include routine water changes in the maintenance schedule helps control nitrate, and could control it adequately on its own.
It may be that the trickle filter could be constructed to deal or help deal with nitrate. The bio media could be sintered glass which contains very low oxygen areas within where bacteria use nitrate. I do not know if this would work as I have not done and have not heard of any experimentation along these lines.
The ability of a good trickle filter was demonstrated when I visited a local fish shop. There were many display tanks full of fish, and these were all on a water circuit incorporating a large trickle filter. The number of fish were high and ranged from small of 2″ or so to large of 12″ plus. The bio load would be heavy but there were no problems, and the water was always clear of toxics. An adequate water change amount was done and nitrate measured between 10 and 15 ppm – not bad for fish only. The coral system ran on a separate loop based on another trickle filter, smaller this time. The water in this system never showed any toxics or nitrate. Again, adequate water changes were carried out. On both systems a protein skimmer was in use.
If the aquarist is incorporating a proper amount of high quality live rock, then of course consideration of a trickle filter is unnecessary, provided husbandry is adequate.
I would see a trickle filter probably being most useful in a fish only system.
I am not trying to tempt anyone to use a trickle filter as the one of choice. Live rock, especially with a DSB, is probably the best filtration currently available. What I am saying is that there are options, and these options can work if, as in all systems, adequate routine maintenance is done.