Personally I dislike canister filters on seahorse tanks. Because they are more involved to clean, you can be more likely to skip cleaning occasionally and end up with nasty bacteria problems. Using the easiest forms of filtration mean you are less likely to avoid it when it needs to be serviced.
Do you not have a large enough biological filter now?
About the ONLY time I would use a canister filter on a seahorse tank would be to add a bit more volume to a small system, and, as a water mover, but I wouldn't have anything in it but water.