Aquarium Filter Designs: Undergravel Filters

Biological aquarium filters, including under-gravel filters, function in a completely different way, and they have a completely different design. The Nitrosomonas and Nitrobacter bacteria necessary for biological filtration will colonize virtually every surface in a tank. However, there is usually not enough surface area in a tank to support populations of these nitrifying bacteria large enough to process all the ammonia that is produced in a typical aquarium.

 

A biological filter deals with this problem by making the natural nitrogen cycle operate more productively.

A biological filter has two main functions. It increases the amount of surface area available to the nitrifying bacteria, and it creates a constant flow of aquarium water over the colonized area.

The most popular kind of biological filter is the undergravel filter. It is relatively inexpensive, reliable, and very effective. An undergravel filter is really just a plastic plate that covers the bottom of the aquarium. The plate has many small holes or slots in it. The exact physical appearance varies from one brand to another, but all work on exactly the same principle.

The undergravel filter plate is covered with aquarium gravel. Located at each back corner of the plate is a lift tube that extends from the plate to the top of the tank.

Aquarium Filter Designs: Sponge Filters

Another kind of biological aquarium filter that is commonly used in aquariums is the sponge filter. Although varying somewhat in appearance, all sponge filters work the same way. Water is pulled through the sponge, which is colonized by nitrifying bacteria. Most sponge filters use a single lift tube and air bubbles to draw water through the sponge, although some large models can use a powerhead instead.

 

There are specific reasons for using sponge filters. For example, if you keep a sponge filter in a tank that already has a biological filter; it will develop its own colony of bacteria. The sponge filter can then serve as an emergency biological filter in another tank.

If you need to set up a small tank to treat a sick fish or a quarantine tank for new fish, the sponge filter can provide immediate biological filtration for that tank. This eliminates the usual break-in period.

For those who breed fish, the sponge filter is also very useful. Often at some point, young fish need to be taken out of the adult tank and put into a fry tank, which contains only juveniles of the same size. Sponge filters provide instant biological filtration for the fry tank.

It is safe for the baby fish because there is no dangerous filter intake, as with power filters, and the micro-organisms on the surface of the sponge even provide an additional food source for the growing fish.

Aquarium Aquascaping Tips

The first consideration when decorating an aquarium, or aquarium aquascaping, is the gravel. The size of the individual pieces of gravel is particularly important if the tank has an undergravel filter. If the gravel is too large, there won't be as much total surface area for the nitrifying bacteria. On the other hand, if the gravel is too small, it will clog with particles easily and restrict the flow of water through the gravel bed.

The best gravel sizes are #2 or #3, which are two and three millimeters in diameter respectively. Although the color of gravel is a matter of personal choice, neutral colors are more natural and do not compete with the fish for attention. They also help to make the fish more comfortable.

You may notice that many fish are dark on top and lighter on the bottom. This is a form of camouflage called counter-shading. The fish will be less visible to predators when viewed from above against the dark bottom of a stream or pond, or when viewed from below against the light color of the sky. For this reason, fish feel more secure over dark gravel.