All I will say is this your entire bottom of the tank 2-3 inches thick is a bio media that holds bacteria when using a UGF.
Heres alittle info from a site I found a awhile ago.
The Undergravel Filter.
These days the poor Undergravel Filter is a continuous topic on many tropical fish forums, and there are many "against" its use, and many "for" its use. My personal opinion is not the question here, but I do in fact use the U/G filters in all my main display tanks, and find them to be trouble free, low maintenance, and they always help to keep my water crystal clear and in good condition.
The greatest impact on the fishkeeping hobby (in my estimation) was the All Glass Tank, but prior to this I would say that the U/G filter was the big leap forward in water control. The old undergravel's were just a series of perforated pipes that were attached together by elbows and tees, and buried in the gravel, then later on came the plate type of U/G filter that we see today, that to my knowledge has been around for about forty odd years. The other filter that has change little in all these years is the corner box filter, and the same concept is used today, as nothing can really improve on it.
So what do we believe...? Is the U/G filter a thing of the past.... Is it really so hard to maintain, and does it really rob our plants of food. I have personally run tanks from 18"x12"x12 to 9foot by 2foot by 2foot for years with U/G filters fitted, and have had no problems whatsoever, neither from blockage, or from lack of plant growth.
To me... and this is just "my" view, the undergravel filter will always have a place in the aquarium, but only under certain circumstances, and it does not suit every occasion and setup. For a planted community tank that is sitting in your front room, then it would be ideal, but on the other hand if that tank was filled with large Cichlids that tend to dig, then you wouldn't use one in this situation, nor would you use one if you only has say a couple of big fish in there with few plants.
Again... we have to look at the "balance" within the tank, for with a tank with few fish and plants, the filter would perhaps rob the plants of food, but the same tank fully stocked, with lots of plants and fish would tend to fair much better. In my tanks you can see very little gravel, as most is covered in a lush growth of plant, and my average tank (1.2x.5x.5mtrs) holds around a hundred or more fish... generally ones that are growing on, but are put in display tanks for a change of scenery. The U/G filters are air driven only, and the odd tank or two has the addition of an external box filter.
Filtering your tank water today is a science, and we see ad's for the latest this and that type filter that will suck the hairs off a peach, and turn your water over so many times an hour, that if you blink you will miss it happening... but is it "really" needed. Again... Some instances do demand constant heavy filtration, but (without additives) a filter can only store so much of the good bacteria that is beneficial to your water conditions, and this of course depends on the amount of area that is provided for the bacteria to live in. Some filters only have a few square inches of "collection area" where others have multiple chambers and additions of what are commonly called "noodles" to help colonize the bacteria. In the U/G filter, the "noodles" are the gravel, and just as each noodle is coated with bacteria, so is each particle of gravel, along with just about everything else in the tank that will hold bacteria. Looking at a four foot by eighteen tank, we would have a filtering area of six square feet, but add this to the depth of the gravel, and you could have several cubic feet of active "collective area", much more than any other type of filter. This area doesn't mean that it is more efficient, or better in any sense, but under the right conditions, the U/G filter has its place.
