The Filter

Mechanical filters sift particles though a fine filter medium, sifting out particles that are suspended in the water. Mechanical filters provide rapid mechanical filtration by using external power filters and canister filters.

Chemical filtration involves the chemical treatment of water to remove toxic substances. You are providing chemical filtration when you add activated carbon to an external power filter.

Biological filtration is probably the best kind of filtration. Although it is easy to set up, it sometimes takes a while to work properly. It is the best system because it establishes natural orders of bacteria and creates the nitrogen cycle, using natural methods. This system utilizes the nitrogen cycle to remove toxic compounds from the water.

An excellent example of a biofilter is the under-gravel filter that draws water through the aquarium substrate. This substrate contains the necessary bacteria to convert nitrogenous wastes to nitrate. What takes the filter a while to establish itself is waiting for the bacteria to establish working colonies in the substrate. It may sound incredibly gross, and will probably look like gross muck at the bottom of your tank underneath the gravel, but it really is a very good way to filter your tank in the most natural way possible.

However, it is important to know that all three kinds of filtration take place no matter how you try to do it. In most commercially manufactured filters of any kind, you can find all three systems at work. For example, the external power filter will mechanically remove particles, chemically remove toxins if the filter contains activated carbon, and biologically convert nitrogenous wastes via the nitrogen cycle in its filter media.

FILTERS 101

Different types of filters for beginners:

  • Box filter—the easiest
  • Under-gravel filter—easy
  • External power filter—moderate

The under-gravel filter takes longer to establish but it really is the best, provided you don't overload your aquarium with too many fish and plants. If you have fish that are going to grow quite large, such as goldfish in a large tank, or oscars, you probably should use an external power filter.

The most common types of filters available to the beginner include the internal box filter, the external power filter, the external canister filter, and the under-gravel filter. Are these the only types available? Certainly not. However, they are the most common. There are others that are better suited to those setting up high-level, exotic aquariums.

For the most part, the filters we'll discuss are the ones you want to use. But which one should you choose? There are many different kinds and configurations by many different manufacturers. Here, you'll find a description of the most common with their good and bad points. You need to pick the one that sounds like the right fit for you.

The Box Filter

The box filter is the simplest and most obvious of all the filters. It is also the most popular of all the filters for starters. It is inexpensive and easy to use. Shaped just like its namesake, the box filter sits inside the aquarium. It is driven by an external air pump.

As air is pumped into a chamber where it mixes with water, the air eventually fights its way up through the filtering medium, and forces the water up through with it. It draws the water through a fibrous filter material and activated charcoal. Layers of filter media provide mechanical and chemical filtration as well as adequate substrate for biological filtration. Because it is driven by air, this filter circulates and aerates the water.

Power Heads

Make sure not to turn the pressure up on your power heads too much, as it may agitate your aquarium environment instead of keeping it healthy. You want some aeration … not a jet stream.

This type of filter is fine for a lightly populated 10-gallon starter set. But any serious aquarist worth his or her salt doesn't use one of these. In our opinion the box filter does not provide adequate levels of filtration for the average aquarium. Aquarists who start with a tank of 20 gallons or more should not use this type of filter system to provide filtration. It won't aerate your water properly and is simply too small to handle the wastes and debris that accumulate in the tank.

For the 20-gallon tank we recommend you start with, the water filtered by a box filter would have to be changed frequently and the aquarium would require too much maintenance. The tank would not be crystal clear and would provide for a bad habitat as well as for poor viewing.

The External Power Filter

While it looks intimidating to the beginner, and costs more than a box filter, the external power filter is the best filter for the beginner aquarist to use. Specifically designed to turn over large amounts of water, these filters, as described in the nitrogen cycle section, provide all three kinds of filtration. The external power filter hangs onto the side of a tank and is driven by its own electric motor. Via two siphon tubes, one for incoming water and one for outgoing water, the power filter draws water in through a medium of fibrous filter material and activated carbon. The water is then pumped back into the tank. Some filters have a system where the water trickles back into the tank rather than flowing back in through a tube.

Though similar in some respects to the box filter, the power external filter is much more efficient and will keep your water much cleaner. The external filters are generally bigger and can go for longer periods without needing to be cleaned. They remove particles and debris better and aerate the water better. Newer models have specialized filter cartridges that make cleaning these filters extremely easy. In addition, various types of cartridges can now be purchased that chemically alter water quality and correct water chemistry problems.

External Power Filter

The External Canister Filter

The external canister filter is often used by more experienced aquarists. However, the canister filter isn't about experience—it's about size. This is a larger filter and is meant to be a filter for tanks of 50 gallons or more. A large, jar-shaped canister, this filter generally sits somewhere not far from the tank.

Like the other filters, it passes water over filter media and activated carbon, but the motor is much more powerful and can manage cycling through a large tank quite easily. Water is drawn by an intake suction tube and sent back to the aquarium through a return tube. These are powerful filters and are not recommended for smaller tanks. Use this with a big tank only.

Canister Filter

The Under-Gravel Filter

I'm Suffocating in Here!

Despite the fact that fish live in water, one of the most important elements to their survival and health is fresh air. Aeration is the act of infusing their water with air. This happens in nature naturally. Streams and rivers have tremendous air to surface ratios, as well as rapids and waterfalls. Your aquarium does not. You should at least have one aeration device in a 10-or 20-gallon tank. In a 30-or more gallon tank (and larger), you should have at least two.

The true aquarium aficionados prefer the under-gravel filter. It is considered to be the best because it provides biological filtration. However, it is deceptively simple. The filter consists of a large plastic plate that is placed underneath the bed of gravel in the tank. The filter pumps air through an external air pump into this bottom plate, mixing the air with water. Bubbles rising through the exit tube trap water, drawing it up through the gravel and through the substrate.

Some under-gravel filters are driven by power heads mounted on the intake tubes. Power heads add extra aeration and move the water more quickly through the filter. Whether a powerhead is used or not, under-gravel filters provide excellent water circulation and aeration. It's so simple, and effective. This filter uses the aquarium gravel itself as the filter media!

Thus, very little mechanical filtration, and no chemical filtration, is involved. And there's no activated carbon! The under-gravel filter relies chiefly on the establishment of a healthy bacterial colony in the gravel. This filter requires a lot less maintenance than the others and is wonderfully efficient.

Undergravel Filter

However, this type of filter has several drawbacks. First, you need to use a certain size of gravel, which limits your aquascaping (aquarium landscaping) choices. Second, this type of filter takes a long time to set up, because it doesn't work immediately. You need to allow the colonies sufficient time to establish themselves. It may be two to three weeks before you can even think of adding fish to your lovely habitat.

There are more pitfalls. If there is excessive debris in your aquarium, these particles can eventually clog the filter bed. They need to be removed often. Some fish, especially goldfish and some catfish, as well as nest-builders, will push the gravel around and upset the substrate. And if you like to keep live plants, say goodbye to your green thumbs, because these filters will play havoc with your plants’ root systems.

All these things make this a difficult system for the beginner. We strongly recommend that you start with an external power filter, and leave the under-gravel filter system for the next stage in your development as an aquarist.

Power Heads

Power heads—you either love them or hate them. Generally they are used to power up under-gravel filters. They do not operate in the same way, however. Instead of forcing air through the under-gravel filter, they force water through the under-gravel filter. They suck water from the top of the tank though the feeding tubes, which causes the same effect in the aquarium's water circulation.

These are small water pumps but they are very powerful, and again, we would only recommend them for experienced aquarists. Many inexperienced hobbyists turn their powerheads up too high, causing too much disruption and making the tank uninhabitable.

Power Head Air Pump

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