Basic Aquarium Filters - Underwater Eco Systems

Basic Aquarium Filters

The Corner Filter
For decades, hobbyists have successfully kept fish healthy and happy through the use of the inexpensive corner filter. Typically, they are clear plastic boxes, which sit inside the tank. An air stone bubbles air through an air lift tube, which forces water through a bed of filter floss or other media. mechanically filtering the water.

Colonies of bacteria build up on the media, providing excellent biological filtration. (It is important to change only some of the media at any given time! This way the bacteria does not get wiped out.) Nowadays people don't use corner filters as much because they're ugly, take up space in the tank, and require a bit more frequent maintenance than other filters. But you can't beat the price.

Another use of the corner filter, that is not really matched by other filter types, is as an impromptu quarantine tank filter. If you have the need to set up a second tank on the quick, you can take some gravel from an established tank and put it in a corner filter, and immediately, you will have a functioning biological filter. This way you can turn a five gallon bucket into a quick and cheap hospital/quarantine tank on a moment's notice.

Undergravel Filters
Fish stores commonly sell undergravel filters to beginners in aquarium kits because they are cheap, and they work for a while.

Undergravel Filters work by slowly passing water through the bottom gravel, which sits on top of a perforated plate. The water can be pumped with an air lift, with bubbles air lifting the water in a vertical tube attached to the filter plate. Also, some people prefer the increased water flow achieved with submersible pumps, called powerheads, attached to the same lift tubes.

Undergravel Filters make good biological filters, because the slow flow of water through the gravel fosters large colonies of beneficial bacteria which neutralize toxic ammonia. The hitch is, that  Undergravel Filters are awful mechanical filters. Fish waste gets pulled out of sight into the gravel. Before you know it, the gravel clogs up. You then have a big mess and a health risk to your fish!

A partial solution to this dilemma is to run the powerhead in reverse, sending water up through the gravel. This technique is known as Reverse-flow Undergravel Filtration (RUGF); conversion kits or special powerheads can be purchased to accomplish this. The intake of the powerhead is covered with a porous sponge which serves to "prefilter" out some of the waste that can clog the gravel. In actually practice, this helps, but is only a partial solution.

If you choose to use an Undergravel Filter, you must regularly vacuum your gravel. Fish stores sell siphon hoses with a wide mouth gravel vacuum tube attachment that washes the gravel during your regular water changes.

If you clean your gravel regularly, and maintain a regular and frequent partial water routine, Undergravel Filters are an economical and effective aquarium filter in freshwater aquariums, and in lightly stocked saltwater fish-only aquariums.

Sponge filters
Sponge filters provide an efficient and cheap form of biological filtration. Water is forced through a porous foam, either by a powerhead, or air bubbling through an airlift tube.

Water flowing though the sponge allows the growth of a colony of beneficial bacteria which neutralizes toxic ammonia.

One style of sponge filter uses two sponges attached to one lift tube. These have the advantage that the sponges can be cleaned one at a time, reducing bacterial loss. Also, one of the sponges can be removed and transferred to a new tank, bringing with it a colony of beneficial bacteria, and thereby "jump starting" the cycling of a new tank.

Some enlightened fish stores sell these double sponge filters to beginner customers when they sell a tank kit. They take one of the new sponges out of the "box" and swap it for a old established sponge in one of their tanks in their store which is carried home in a plastic bag.