Undergravel filter help

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Fish Fanatic
Aug 9, 2004
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Hi, I have just bought a 20 liter (55 gallons) hexagonal fish tank. The aquarium centre gave e everything I need, I have an undergravel filter and an air filter that clips into it, and this has an external Rena 100 pump connected to it. Right what I need to know is, under that undergravel filter can any gravel go in there or does it have to be completely clear as I am having trouble keeping out small bits of the gravel?

Also loads of bubbles shoot up inside the air filter and out the top (which is under the water), should any be coming out of the bottom of the filter and out as I don't see any bubbles?


The way ugfs work is somthing like this

the air stone (inside the tube) lifts the water by convection this in turn brings more water through the gravel, thus pulling the waste into the gravel where it gets trapped.
so you should only see bubbles comming from the rising tube.
the underneath of the ugf should be as clear as possible althought a small amount of gravel under it will not affect the process.

Ugfs are considered by many to be the most basic form of filteration now a days and most aquarists use internal or external filters

Hi, would this form of filtering be ok for my size tank (5 gallons), or would you suggest taking this out and putting in an internal filter?

If so would I need a new pump etc or could I leave the setup as it is then just add another internal filter?


for a 5 gallon it would be ok, you just need to
keep on top of the weekly housekeeping
Hi, I have the current air filter fixed into the UGV, the blue air stone thing at the filter is pushed to the botton on the glass. I see loads of bubbles going up into the filter and out the top, but I never see any coming through the gravel, should I?

Kind Regards

UG filters are very basic but very effective. You just have to keep on top of your gravel cleaning.

You should not see any bubbles coming from your gravel, the flow of water is down through the gravel and up the uplift, drawn by the air from your difuser, and out the top.

Thanks to both of you. That diagram is perfect, I really appreciate your help.
The best way I have seen is the slope effect starting with
1" in front going to 2.5" at the back. :thumbs:
Hi Andy

Here are some good tips for an Undergravel filter.Put about two inches of gravel over the top of the filter plate. Keep fish stocks to the minimum for the size of your tank as UG filters are not as eficient as modern internal or external filters . The other way to run a UGF is to fit a power head to the top of the tube coming from the UGF, this replaces the air stone and pump . When doing water changes put the siphon tube down the UGF tube and take water out of the tank this way, it takes some of the sediment that gets trapped in the space created by the Filter out of the tank.......better still buy an external filter!
UGF are very efficient filters--they provide more surface area for bacteria to colonize that just about any other appropriately rated filter. For smaller tanks, with light bio-loads, there is no reasonto change to a different filtrations--you will actually be reducing the size of the biological filtration, especially in trying to find a power filter rated for a small tank. Running a powerhead on a 5 gallon tank is serious overkill--you'll end up with way more current that is needed.

You will need to maintain the gravel bed by cleaning it (siphoning it, no need to remove and rinse) weekly. This will not disturb the bacteria in the slightest, as they are firmly attached to the gravel--you'll just suck up the solid wastes that have become trapped in the gravel.

About a month ago, I started my first tank (20 U.S. gallons) with an UGF. Soon after, I read all the information on this great site about the superiority of an external filter such as a Penguin Bio-wheel. I followed their advice, and switched out. I waited about 4 days before I began to wonder if I should take out the UGF. Once again, I turned to the forums and the advice was to indeed take out the UGF. Even though it had only been about 4 days, when I removed the filter I smelled the classic "rotten egg" smell which indicated the presence of anerobic bacteria (basically the bad stuff.) It was amazing how much "junk" had fallen underneath the plate. (I had three platies in the tank for a total of one week before removing the plate.) So, my advice is to remove the UGF as soon as possible and clean the gravel well. If you have any fish, bag them and make sure the sediment and "junk" settles before releasing them. I actually lost a fish because I didn't wait long enough for the sediment to clear.

HTH and Good luck!


Would I have to get rid of my air filter that plugs into mu UGV filter?

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