A wet/dry filter, also referred to as a trickle filter or a bio-tower, is an aerobic filtration method. For those of you that may not understand the term aerobic, it means occurring or living ONLY in the presence of oxygen. In other words, it can only work when oxygen is present. For this type of filtration, the more oxygen saturation it gets, the better it functions. Water is pumped from the aquarium, then by means of a drip/trickle plate or rotating spray arm the water is dispensed or "trickled" down over and through a biological material source contained in the wet/dry filter chamber, but not before the water is pre-filtered by means of mechanical filtration, which can be accomplished with the aid of a protein skimmer or by placing a pre-filter material such as filter floss, a filter sponge, or micron filter felt on top of the drip/trickle plate area. When the water falls through the holes of the drip/trickle plate onto the bio-media, this allows for aggressive oxygen saturation of the water. Remember, aerobic! The clean filtered water is then deposited back into the aquarium either directly, or first into a sump or some type of water containment area and then returned.
As far as how it fits into the scheme of things as far as filtrations goes remember this, many will start their tanks with that natural filtration of live rock and live sand, two basic natrual filtration methods. Many believe that more is better but you can't have a tank full of sand and rock and a few gallons of water and hope to see fish and corals as well, so they add a tank that many would call a refugium, with maco algea, more sand and more rock. Many hobbiests prefer a more natural approch so they will take out the bio balls (which many villagers feel are a nitrate factory as they get out their pitch forks and torches and blame everything on the poor misguided bio balls). It is the pre filter or filter floss that may be the real danger here as it will trap and filter out many of the plankton that would be a benifit to the system and corals. All depends on what your goals are.
I use a wet/dry and maintain my bio balls, I keep my filter floss clean. I have also been left wanting something. If I had the time I think that I would setup a fuge. Many of us however do not have the time, space, funds for such projects. so the wet/dry is all we have. If maintained properly it will serve you well and you can succesfully have a good clean tank.
Thomas
