How to maintain an UG filter;
use a gravel vac at every water change.
every 3 months put a hose down the lift tubes and siphon water and muck from under the plates.
about every 6 months -1 year get in there and really stir up the substrate to break up any compacted spots and keep it relatively clean.
also gravel choice is important. You don't want gravel that is too large as that will allow too much muck to settle in the substrate and under the plates. The common gravel sold in most lfs
is good, not too big, not too small.
Avoid sand as it compacts to easily and prevents a good flow.
the caveats to using an undergravel is in order to use carbon or other chemical filter media you need to use a secondary filter.
and if not maintained by vacuuming and siphoning from under the plates, they can be disaster waiting to happen.
Well maintained UG's are one of the best biological filters that can be used.