I have used undergravel filters(
UGF) in the past and use Hang on the Back (
HOB) filters now.
The UGF pulls all the gunk(excess food, poop etc) under the plate and it stays ther building up. Beneficial bacteria grows in the gravel and on the tank decor and takes care of the
ammonia and Nitrites. Nitrates just continue to build andeventually get extremely high. If the tank is planted that will take care of a good bit of the Nitrates but now you've got plant roots tangles up growing through the UGF plates. when I took mine out a year later there was about a 1/2 inch of sludge and dead plant roots under the plate. It was a real mess.
HOB filters have BioBags or sponges of some kind in which beneficial bacteria grows. When they get dirty the filter media is taken out and rinsed in used tank water to get the gunk off then reinstalled in the filter. The media should never be thrown away until it's totally ragged. The Hob is so much easier to clean and Nitrates don't build up like they do in a UGF.
There are also canister filters that are equally easy to clean and use. They are hidden away. For a really large tank they can't be beat.
Carol
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