Filtration: A Primer

One of the most vital pieces of equipment for aquariums are filters. In the closed system aquariums are, filters, which have been around since the start of the tropical fish hobby, contain media to be colonized by Nitrifying bacteria. The bacteria, of which there are several type of, metabolize, the Ammonia (NH3) generated by fish from their urea, with the waste product Nitrite. Another type of Nitrifying bacteria metabolize the Nitrite (NH2), resulting in Nitrate (NH1).

And there the bacteria stop, but there are low Oxygen bacteria that metabolize the last Hydrogen atom in Nitrate, resulting in Nitrogen gas, which you and I, and every air breathing organism on the planet, breath. Thus there are filters, and filtration types, that can keep Nitrate in a tank at zero.

And there are several types of filters available to the hobbyist. As in many products, some are better than others.

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The very first filter I was aware of was my late father's air driven box filter which he filtered his 29 gallon community tank with. A box filter is simply a clear plastic box, slots in the top, with carbon at the bottom of the box and filter floss on top of it. It worked because the bubbles from the air generated by the air pump pulled Oxygen rich tank water through the filter, meaning it was quickly colonized by Nitrifying bacteria. Though box filters are a thing of the past, they do work, but their general small size, and small amount of media, means they are limited in their ability to filter.

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Under-gravel filters were in favor for some time. An under gravel filter is simply two ridged plastic plates, with slots atop the ridges, and two clear plastic cylinders rising from the corners of one side. The plates are placed at the bottom of the tank, and substrate placed atop them, Usually power heads are used to pump tank water into the filter.

Under-gravel filters began falling out of favor in the late 70's as they are easily clogged with fish mulm. Back-flowing the filter regularly can help, but one most have another filter connected to the tank to remove the now suspended mulm.

Under-gravel filters provide only biological filtration, as the carbon-filed cartridges that come with some of them do little work.

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Power filters started appearing in shops in the years after WWII. A power filter is hung on the back of the tank. Tank water is pulled into the filter by what is called an impeller, which is a spinning object that spins fast enough to create suction. The water goes into a reservoir in the back of the filter and flows through the media, which is usually a relatively flat object placed in such a way that the water must go through it. the filtered water returns to the tank, the vast majority by waterfall.

A power filter is limited by the type of cartridge they use. Because of the relative thinness, there's little room inside for carbon or other media. Though the better brands of power filters are quite capable, none can hold much media.
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Wet-Dry Trickle Filters came in vogue in the 1980's.

A Wet-Dry filter is usually attached to a sump. Tank water is pre-filtered to keep debris from entering the filter, and it trickles down through a fibrous media, bathing the Nitrifying bacteria in a high level of Oxygen, meaning they are extremely efficient at keeping Ammonia and Nitrite at zero in aquariums with large, messy fish.

The downside of West-Dry filters is, in those large aquariums with large fish, the amount of Ammonia the fish produce means high Nitrate is always a problem, with 150 ppm or higher common. A keeper with large fish in large tank must do large partial water changes every week to prevent the high Nitrate from damaging the fishes. Because of this, Wet-Dry filters have gradually fallen out of favor since there are more efficient filters out there.

In smaller tanks with smaller fishes, Wet-Dry filters work just fine, but partial changes must be weekly without fail to dilute the Nitrate.
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Canister filters, though a good deal more expensive than hang-on-back power filters, are the most efficient filter one can buy.

Simply because they hold so much media. As most brands use baskets or similar, one can use any media they like in a canister filter. And since canister filters have much more powerful motors, one can hide a canister filter much easier than a person can a power filter. Since I dislike the appearance of equipment in or around a tank, all my tanks use canister filters.

A canister filter is simply a plastic canister with the motor and inflow and outflow valves at the top of it. Hoses reach up to the tank, and usually have a bracket to hold the hoses in line. Plastic inflow strainers and outflows top the hoses. Canister filters have far higher gallon-per-hour rates than power filters are capable of.

Most have sponges or similar to prevent debris from getting into the media chamber(s). The water circulates up through the media, and the filtered water returns to the tank.
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Other types of filters

Fluidized Bed

A fluidized bed filter is simply a acrylic tower, with two connected chambers and inflow and outflow connections at the top. Sand or other fine-grain media is placed within it, and a prescribed amount of tank water is pumped into the filter until the sand is in moderate suspension. Nitrifying bacteria colonize the grains, and since they are continually bathed in Oxygen-rich aquarium water, the bacteria are quite efficient at metabolizing Ammonia and Nitrite.

Fludized bed filters are rarely used by themselves, and are most usually plumbed downstream of a main filter.


There are many types of reactor filters, but all are designed to remove varied dissolved excesses in aquarium water.

Nitrate reactors have been around a long time, and have trickled down to the home hobbyist from the aquaculture and public aquarium sector. Nitrate Reactors are plumbed 'downstream' of main filtration units.

Nitrate is the end product of Nitrifying bacteria that exist in the relatively high Oxygen levels of the average aquarium. A Nitrate Reactor removes said Nitrate, which can be a problem in tanks with large or messy fish. Properly used, a Nitrate Reactor will keep Nitrate at zero.

There are several types of Nitrate Reactor. All are acrylic towers with inflow and outflow valves at the top. Most need a relatively slower flow for more time for the bacteria to do their work. The reactors work in marine tanks as well as freshwater aquariums.

The first have inert media, usually plastic spheres that the bacteria colonize. In such filters the bacteria have to be fed regularly with dilute alcohol (about 1 percent).

Better is a Nitrate Filter with spheres designed to keep the bacteria fed mixed with the inert spheres, meaning less work for the user. The downside is one must purchase the feeding spheres regularly, the rate of replacement depending on the level of Nitrate processed.

Using Sulphur balls as media provide both colonizing space and food for the bacteria, but that type of Nitrate filter is mostly used in Europe, because the sulphur media is difficult to find in the US.

Other Reactors

Phosphate, at levels above the natural level of .01-.02, can fond algae, especially in marine tanks, which is why there are so many organisms sold to reef keepers to keep algae in check. Phosphate reactors, of which there are several, usually use Iron Oxide pellets within them to reduce excess Phosphate to natural levels, preventing it from becoming algae food. There are several brands of pellets designed for Phosphate Reactors, but all are based on Iron Oxide pellets. Most use a valve to slow the flow through a Phosphate Reactor to give the bacteria on the pellets more time to remove the excess Phosphate,

Phosphate Reactors, though mostly used by marine aquarium keepers, work just fine in freshwater aquariums.

Other media can be used in such reactors. One of the most common remove high silicates.

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