Undergravel filters have the lowest profit margins. Plus at some point we started adding powerheads to them and ruined them for ever. This craze with flow through media popularized by canister manufacturers as a means of estimating tank size by their cylindrical tubes could handle or was it HOB?I own just about every book I could get my hands on, there is a lot to be learned from them!
Especially that undergravel filters are the standard for saltwater.