IMO, mechanical filters are not needed in a dwarf seahorse tank and I have none in my three dwarf tanks myself. (I use live rock for biological filtration only)
Because dwarfs only eat live food, primarily enriched (Dan's Feed) baby brine is the most convenient, and the fact most of them don't "hunt" food like their larger cousins, food density at time of feeding has to be intense or they slowly starve to death.
After feeding, you remove the excess so that they won't be feeding on less nutritious live food at next feeding or in between feedings.
If you put a strong light next to a convenient spot on the tank the excess artemia will congregate there so you can vacuum them out, adding new salt water to replace what has come out.
That, and in addition to the "wiping down" of interior surfaces and water change to remove residue about once a week (frequency depends on stocking density of dwarfs) will change enough water to not need filtration.
Using a sponge filter can sometimes trap the smaller dwarfs (and sometimes larger ones) ending up with mortalities if not caught in time, unless the air flow for the sponge is quite low.
Low air flow means less turbulence at the waters surface so my preference is to not use the filter but instead use rigid open ended air line to get the surface motion needed.
