a 30 hex should be fine for a pair of horses. The hard part is the aquascaping. Tonga branch works well, but its hard to come by.
As far as filteration, take out the undergravel filter, and add either a hob or canister. you can do both, turning the hob into a fuge. You should also add an open air line. No air stone.
T
Pictures of the tank??
What species were you thinking of. You could do fuscus, barbs, erectus - Reidi might be pushing it for size.