by Jack MacDowall

Created on: January 17, 2007   Last Updated: May 10, 2007

There are many different kinds of filters on the market. They all do the same things: keep the water clean for the fish, and keep the water clear, so you can see the fish. But the array of choices can be confusing to a beginning fish keeper, as various types of filters do not, in fact, do the same thing. A Under Gravel Filter is not intended to be a mechanical filter, but a biological filter. A carbon insert is designed to be a chemical filter, not a biological filter. Confused? OK, let's break this down.

Filters work by moving the water through a media that essentially strains the dirt out of the water. Classic "bubble-up" type filters rely on an air pump to create a stream of bubbles that lift the water out of the filter and create a flow through the filter. The water is filtered by the floss and then passes through charcoal that absorb the impurities. After a while, some bacteria start to grow in the floss that can break down the ammonia and nitrates in the tank water, so the filter will do additional duty.

Some filters hang over the back of the tank and have a siphon tube that keeps the water coming in. A small motor moves an impeller that pushes the water through the filter media and back into the tank.

An Under Gravel Filter uses the fish tank gravel for the filter media. The water is drawn through the gravel, that is supported by a plastic plate with slots for the water to pass through. The water continues up a "lift tube", drawn by a powerhead or by an airstone bubbler.

Experience will dictate what works best for you. The "hang-off-the-back" type seem to be the most popular these days. It's very easy to replace the filter media bag, without even getting your hands wet. Don't be tempted to buy a huge filter with the hopes that your water will be crystal clear. It will only turn your tank into a wave-pool. Buy a filter that is rated to your size tank!

But what are the real differences in filters? Three types of filtration need to be done to your water:

Mechanical - this is just a matter of getting the big chunks out. Filtering the water through a sponge or some filter floss gets the solids out of the water. Most filters do this with the exception of a under gravel filter. Filtering the water through gravel does a poor job of removing all but the largest particles. UGFs do make great biological filters because of all the little spots for the bacteria to hid out in the gravel.

Chemical - charcoal is the best here. Charcoal is great at absorbing chemicals and toxins in the water moving past it. Charcoal is what you find is the water systems for drinking water. Not that you'd wanna' drink the water coming out of your chemical filter, but it sure keeps the fish healthy.

Biological - this is what we looked at on the cycling page. Biological filtration is moving the water through a medium with lots of surface area to which are attached a huge colony of bacteria who's mission in life is to break down the poisonous ammonia and nitrates in the water.

I find the best solution is a combination of filters. I start with a hang-off-the back filter. It has a cloth bag full of charcoal. The charcoal does the chemical filtration. The bag does the mechcanical filtration. The two of them seem to provide a great place for my bacteria colony. I also like to have an UGF going. My tanks generally do better with a UGF with a powerhead attached to the top of the lift tube.

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