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Undergravel Filters

Guest fantail_luver

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Guest fantail_luver

I was just wondering, are undergravel filters good enough for goldfish? or if I used one (by itself) would it not be good enough?

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By itself, no, an undergravel filter would not be enough for a goldfish tank. I also do not like nor will I use an UGF in any of my tanks. That said - there are those who do, and with the proper care, they have great success.

An undergravel filter relys completely on bacteria to break down all the waste that is collected in a tank - it is a biological filter - no mechanical or chemical filtration is included. The waste is sucked down through the gravel and gathers under the plate that is at the bottom of the tank. The hollow space beneath the plate collects the water and it it there processed.

In a lightly populated tropical fish tank, it can be a relatively easy thing to balance the amount of waste an UGF can process with the fish load in the tank. In a goldie tank, the waste that is created is maximized - that is what goldies do. It is a rare UGF that would be able to handle that amount. If there is insufficiant oxygen under that plate, there will be anaerobic activity breaking the waste down, and this can allow sulfide gas to accumulate. If released into the tank, it can kill your fish in a matter of moments.

The best description of UGF I have seen comes from Rick at Goldfish Connection. Here is a quote from him:

"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."

Here is the link to the entire paper he has presented:


I, personally, would think that most people would be far better off having HOB filters, or combined HOB with cannister filters for their tanks. These are easier to service, easier to view and watch for problems or buildup or whatever. There are many options that you can have with a HOB or cannister - from adding carbon or removing carbon, adding additional biomedia, adding polishing filter fibers, etc. An UGF is what it is......


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