How often should the water be changed? It all depends on what you are feeding your fish and also the size of the tank. Smaller tanks need frequent water changes as they get polluted faster. We usually have daily water changes to maintain good water condition.
Frequent water changes also causes the water pH to be more alkaline which is good for the Flower Horns.
An overhead filter is sufficient in terms of filtration but some people have used expensive external filtration units to maintain water condition.
In the end the best filter is still regular water changes.