The main drawback to an undergravel filter is that you can get deadspots in the substrate which is basically dead bacteria(I think).The bacteria will pollute the water and kill the fish if not fixed quickly.This usually occurs when you have large rocks that prevent flow through the gravel/sand.They also are not good for plant growth.Plus, with so many choices on the market, why not try something a little more modern if you can afford it.That said, I used an UGF for a few years to breed feeder guppies in a 10g and the water was always great.But it was just pea gravel w/a few plastic plants so there was good flow.