IMO, you would have a few problems in that endeavor as the refugium temperature would be higher than the recommended temperatures of 68° to 74°F for seahorse keeping on top of the already mentioned fact the seahorses would quickly decimate the pod population.
Also, as I assume the refuge is hooked into another system, you may have problems with exposure of seahorses to pathogens from other system fish and seahorses have problems dealing with pathogens they haven't grown up with.
For a stand alone tank the 15g would be too small as the recommended tank size for most standard seahorses would be 29/30g for a pair, and an additional 15g for each additional pair.
Higher volumes and lower temperatures work much better because of the eating habits of the seahorses that produce "dirty" water that promotes nasty bacteria cultures that end up causing infections/deaths. They generally need larger, more frequent water changes than a reef tank, and, better husbandry to remove anything that gets trapped that can provide a bed for the nasty bacteria.
Any mechanical filtration should be cleaned at worst, once a week, and better for more frequent.
