I have mixed feelings on undergravel filters. If you clean them often and correctly, they work fine. Most people tend not to get them cleaned, so they end up being a dirt repository that tends to throw their PH balances in the tank out the window.
There is a tecnhique called a reverse undergravel filter where you are pumping water under the gravel and out, rather than sucking dirt into the gravel, which is promisng as using the gravel as a benificial bacteria growth surface, but I've never really felt it provided enough benifit for the amount of equipment required, and it does limit your other equipment choices in a planted tank environment.
I have a needlessly elaborate and overly complex filtration and life support system. Mainly because thats one of the areas I enjoy tinkering with. It tends to work pretty well though for a larger tank.
I started with a largeish custom acrylic tank built to my specifications. It is extra tall to accomodate angel fish, and rigged like a saltwater reef tank, with a drilled overflow and a durso standpipe in the back right corner. The water return to the tank is via a diffuser spraybar, to spread current around all the plants rather than just one spot. The water travels down the drain pipe underneith the aquarium to a custom designed sump / refugium. in here, the water travels through one major chamber of filter floss media to keep the particles out of the water, then over a bunch of carbon packs, a bunch of "bio balls" for benificial bacterai growth, and over a bio wheel. From there, it enters the refugium section, where I grow a ton of mosses and harvistable plants, and plants that tend to fix trace minerals that are harmful to fish, like baby mangroves. Those get thinned out every so often to remove toxins. From there it returns to the tank via pump. What this arrangement does is essentially double the surface area of my tank for oxygen exhange, without adding a huge amount more space to the area my tank takes up. I also have the sump set up on a timer, so that whenever the lights in the main tank are on, the lights in the sump area are off. In theory this causes the sleepint plants to produce oxygen and use c02, which is used by the plants up top. when the lights reverse, the oher plants take over. so essentially one set is always feeding the other set.
All the water from the sump also travels through a UV steralizer, which is necessary if you plan on keeping freshwater clams, but a good idea nonetheless.
The area used by the undergravel filter in most tanks is taken up by a plant root heating cable, which promotesr root growth. I have also deliberatly seeded the gravel with a few hundred trumpet snails http://www.yamatogreen.com/MalaysianTrumpetSnails.htm which eat all the decaying matter that makes it into the gravel. The gravel bed is extra thick, to accomodate the needs of the plants, but also to act as a de facto berlin filter.
My goal is to make the mechanics and bilogical components clean and filter everything without needing carbon or these thingies: http://www.marinedepot.com/Seachem_Purigen_100mL_Bag_Carbon_Replacements_Resin_Chemical_Filter_Media-Seachem-SC3231-FIFMCHRM-SC3231-vi.html
I think it can be done, but I'll need a larger refugium on future projects, or perhaps a supplemental hydroponics garden that I can run the water through
