None of the fish in this tank do well in tanks with a lot of water movement. The Seahorse, in particular, is not a strong swimmer and will find it extremely difficult to feed and move about in a tank with anything more than a very light current. Therefore, the filtration system for this tank must be efficient without creating excessive water movement.
Seahorses need a holdfast to wrap their prehensile tails around for stability. In the wild and in your tank, they spend a lot of their time just "hanging out". If they have nothing else to hang on to, they will end up parked in the corner of the tank with their tail wrapped around the heater, which can be dangerous for the seahorse.