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Filtration Options for Goldfish

"Filtration Options for Goldfish Aquariums"

By: Rick G. Copeland

If you're ever among a group of goldfish keepers and you want a little
excitement just bring the subject of filtration. To be on the safe side,
you might want to make sure that there are no sharp objects in the room
before the subject is broached.

There are many options and combinations of options available when selecting
filtration for goldfish tanks. My hope is to present these options so that
you can make a logical choice on what will work best for you.


Before discussing the different types of filter let's look at the basic
goals we're trying to achieve with our filtration system.

Mechanical Filtration -- The removal of the solid detritus (fish poop) and
uneaten fish food from the aquarium. To accomplish this the filter(s) must
generate enough circulation so the solids don't settle to the bottom. The
media to trap these solids can be pleated paper cartridges, Dacron floss,
filter sponges, etc. This media is rinsed or replaced at regular intervals.

Biological Filtration -- Provides a medium in which nitrifying bacterial can
flourish. This media varies with different filters. The more oxygen
supplied to this nitrifying bacteria the more efficient the Bio-filter.

Chemical Filtration - The use of chemicals to absorb impurities or alter the
chemical makeup of the water. Some of the chemical filtration products are
ammonia-absorbing resins, phosphate-absorbing iron oxides, charcoal, and
various chemically treated filter pads. These types of media must be
regularly recharged or replaced. Personally I don't use chemical
filtration. My feelings are that with good mechanical filtration, healthy
biological filtration, and weekly water changes, chemical filtration is


POWER FILTERS - One of the most popular types of filters. Power filters
hang on the back of the tank. They can provide mechanical filtration,
biological filtration, and chemical filtration.

There are many brands of this type of filters. Different brands have
varying types of filtration media.

My personal favorite is the type of power filters that use sponges as the
filter media. These filters will also accommodate resins or charcoal for
chemical filtration. Instead of using chemical media I double up on the
sponges. By alternating which sponge gets rinsed out, established
nitrifying bacteria will be left undisturbed on the other sponge.

Certain brands on power filters have bio wheels attached for additional
biological filtration. Bio wheels are addressed as an individual topic
later in this article.

CANISTER FILTERS - Canister filters are another type of power filtration
that has been available to fish keepers for some time. One advantage with
canister filters is that there is more room for media allowing for more
combination of filtration media types. Also, canister filters are more
efficient in moving the water through the filtration media.

Optional media for canister filters includes sponges, Dacron floss, pleated
paper cartridges, ceramic and plastic shapes for biological growth, and of
course chemical media.

One disadvantage to canister filters is that they can be difficult to
service. Disconnecting them from the water supply and return lines can be a
wet experience if you don't have shut off valves installed or forget to use
them. Another disadvantage is that should circulation in a canister filter
stop the healthy aerobic bacteria die quickly. If the filter should stop
for some time then restart a toxic flood of anaerobic bacteria by-products
will be flushed into the tank. This can happen with any type of filter but
since the canister is a sealed system it will go anaerobic in a shorter
amount of time.

SPONGE FILTERS - Sponge filters are air driven filters that draw water
through a sponge. Sponge filters are excellent for biological filtration.
They are poor for mechanical filtration. The exception to this is when
using them for fry tanks. Then, with regular rinsing of the sponge, a
sponge filter will provide mechanical filtration without endangering the
baby fish. Sponge filters have no chemical filtration capability.

By far the best sponge filter is the cylindrical type mounted on a weighted
pedestal. These filters have large sponges that can be stacked to increase
bio-capacity. A power head can be used to drive them. Attaching the power
head to the filter is somewhat awkward so I prefer to use air to drive these
types of sponge filters.

INTERNAL CARTRIDGE FILTERS - These filters are driven by submersible water
pumps similar to power heads. They use sponges for mechanical and
biological filtration. Some of the larger models can also accommodate media
for chemical filtration. These filters are well suited for smaller tanks
and quarantine tanks.

UNDER GRAVEL FILTERS - The under gravel filter consists of a slotted,
plastic plate that rests on the bottom of the tank and gravel is placed on
top. Water is then drawn through a 1-2 inch layer of gravel either by air
driven lift tubes or power heads.

This filter is by far the most controversial among goldfish keepers.
Problems can arise with under gravel filters if the gravel is not regularly
vacuumed and they are used for mechanical as well as biological filtration.
I view the under gravel filter as biological filter only. A power or
canister filter must be used in conjunction with an under gravel filter in
order to provide effective mechanical filtration.

When setting up an under gravel filter the proper flow rate must be
determined. For an under gravel filter to function properly a flow rate of
1-2 gallons per minute per square foot is required. For example a 55-gallon
tank has roughly four square feet of bottom area. This would require
240-480 gallons per hour of flow rate to ensure proper circulation through
the gravel. Using power heads to drive an under gravel filter is highly

Regular vacuuming of the gravel is required to ensure that no accumulated
waste builds up and no pockets of anaerobic activity is allowed to grow or
sulfide gas to accumulate. Also, every 6 to 18 months the gravel and filter
plates should be removed to clean any built up waste.

REVERSE FLOW UNDERGRAVEL FILTER - This filter is basically the same as a
standard under gravel filter except the water flows from under the filter
plates, up through the gravel. This is done by attaching the outflow of a
canister filter or power head to one a lift tube of the under gravel filter.
All other lift tubes are removed or sealed off. It is important that the
water pumped under the gravel be pre-filtered so that waste will not
accumulated under the filter plates.

The advantage of this type of under gravel filter is that the solid waste is
kept above the gravel where it can be removed by a canister or power filter.
Regular vacuuming of the gravel is still recommended to clear any channeling
or dead spots that may form in the gravel.

WET/DRY FILTERS - Is one of the most efficient biological filters is the
wet/dry filter. This filter was developed for saltwater reef aquariums for
optimum water quality. The concept is that nitrifying bacteria thrives in
an oxygen rich environment. This oxygen rich environment is pre-filtered
water dripping over various plastic shapes (bio-balls).

One concern with using a wet/dry filter on a goldfish tank is to provide
enough circulation to ensure adequate mechanical filtration.

There is also an overhead wet/dry filter that is gaining popularity among
goldfish keepers. These filters rest on the top of the tank and are
designed for aquariums with the capacity of 30 gallons or under.

BIO WHEELS - Bio wheels are cylinders constructed of pleated paper. The bio
wheel turns with the flow of water produced from the outlet of a power
filter, canister filter, or a power head. This action results in an oxygen
rich environment for maintaining nitrifying bacteria. The extra oxygen
added to the water by bio wheels is a welcome addition to any goldfish tank.

FLUIDIZED FILTERS - Fluidized filters consist of a container containing sand
that pre-filtered water is circulated through. The sand becomes the media
on which the nitrifying bacteria will establish. The advantage of this type
of filter is that while the sand is in motion the nitrifying bacteria is
constantly exposed to nutrients and oxygen.


Filtering a goldfish tank requires both mechanical and biological
filtration. When deciding on a filter design for a goldfish tank I prefer
not to put all my eggs in one basket. That is to say I prefer having two
types of filtration on each tank. For tanks with gravel I use an under
gravel filter combined with either a power filter or canister filter. For
bare tanks I use sponge filters in combination with power filters or
canister filters.

When sizing your filter requirements remember that goldfish produce a lot of
waste. To be on the safe side I would use twice the filter that the
manufacturer recommends. For example, if a filter will handle up to a
125-gallon tank I would add two such filters to a 125-gallon tank.

While you do want enough circulation to remove all detritus and uneaten food
you don't want to overdo it. Fancy, round-bodied goldfish are not the most
efficient swimmers. When positioning the outflows for you filters, be aware
of what flow patterns you are setting up. Don't hesitate to add sponges to
out flow tubes to help break up the flow of the water. Also, try to keep
the surface of the water agitated. This is where the most of the exchange
happens between air and water.

In summary when selecting your filter set up think about how much work it
will be to maintain. Also, take a look at what your local fish store uses.
Ask fellow fish keepers. Then determine what will work best for you. Keep
in mind all filtration types require regular attention. If ignored, any
filter can become a ticking time bomb.

Rick Copeland - 05/25/03 - Viewed 59199 times.

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