Aquarists, what should I supplement my filtration with?

I have a 35 gallon with which I use an undergravel filtration system (I like undergravel because it’s self-contained, nearly silent, no exterior containers or lines to leak, and the live plants love it). I’ve never had in problems in the past, but for this tank it just doesn’t seem to be enough (and my water wants to be pretty acid). I’m thinking that I’d like to try an additional filtration system but I’m seriously out of the loop of current tank technology. Any recommendations?