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How to tell if an undergravel filter is clogged. I have got a lot of debris in the bottom but...


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Hi :

I need your help and/or opinions.

I have got a 40 gallon breeder tank with an UG filter.

Two uplifts with two 130 gph powerheads.

The fish are ok.

The water is clear.

I only have anubias that are ok  and some vallisneria that do not die but do not grow or propage neither.

I have tried to run the powerheads to full power, half power and minimun power with no difference.

The thing is that I have got a lot of debris/dirt/whatever you wanna call it in the bottom of the tank and, when I change some water and vacuum the stuff, I notice that the gravel is compacted, not loose. Just the surface, for I do not deep vacuum.

I think the UG is not clogged because I think the powerheads are working fine. The water flow is adequate.

But then I wonder why all that debris does not go through the gravel as I think it should.

I have tried to reduce the amount of food, but it is not helping.

The size of the gravel is medium. 05-08 mm maxium.

Do you think the UG may be clogged?

Would you increase or decrease the water flow?

What am I doing wrong?




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It's hard to clog a UG filter. There's so much surface area that clogging it is pretty tough. Portions of it can clog, but to clog the whole thing is challenging. You don't need a lot of flow with a UG filter. Media contact time is something of a factor with biofiltration. Slower water flow gives the bacteria more time to extract the nutrients from the water. The most efficient forms of biofiltration tend to have slower flow and more contact time with the media.

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For a test try running with an air pump and air stones. Drop the tank water level to below the uplift tubes. If water "pumps" out then your substrate is not clogged. I don't believe you can do this test with powerheads because the head may run dry. Below is a video of my 50+ year old UGF pulling water through the gravel and pumping it out above the water level.





Edited by Sweet Fish O Mine
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The gravel is the primary filter.  Debris must break down and degrade before passing into the UGF.  I'm a firm believer in powerheads.  The downside is that just like air driven filters, they will draw more water from any point where there is less restriction.  You have about 640+- inches of surface area. There will be less water exchanged anywhere there is a buildup of debris, above or below the plates.  I would recommend the occasional deep gravel vacuuming.

It is possible that if you restore flow to a part of the UGF that has been clogged for a while that you could have a debris blowout🤯 best described as a brown cloud. I believe this to be more visually alarming than dangerous.  Your UGF will also clean this up in a few minutes/hours.

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