What exactly does an undergravel filter do? does it filter anything? There are tubes coming up from them, and air hoses going in the top, but do they actually circulate out the crap under your gravel?? It doesn't make any sense to me really.
They suck all the debris down into your gravel, and use the substrate for bio filtration. Down side is lots of crap wedged in your gravel for skyrocketing nitrate levels (unless you vac religiously) and a major pain if they get clogged/need maint.
They work with other filters they are biolodgical filtration. Your tank will be more stabil in those aspects. I always vac my gravel anyways. Why whould i do it just for a ugf. As for nitrite levels i have never seen them high with mine. Maybe if you never vac.
if you have never seen an UGF, you will see that it has a set of raised plates with a "Grill" like pattern on the bottom that covers the entire bottom of the aquarium. it keeps the gravel out and allows about 1/2" free space under the gravel, hense the name UGF. the tubes have air line going all the way the bottom of the tubes. this creates bubbles in the small space of the tube forcing the water in the tube to rise up with the bubbles and exit the top. the water that is displaced in the tubes is replaced by the water from under the gravel.
the whole point is to have a good water flow with high 02 levels to reach your gravel where the bacteria can utilize the 02 and provide good bio-filtration. It can get clogged with waste if you don't vaccum it though. Hope this helps
I used to run a UGF along with a hang-on and I must admit, my water was clear. I would syphon the gravel with every WC, but crap would still accumulate underneath, and you wont know until you take it out and see for yourself.