I kept my first horse, Valiant, in a 90 with a fire goby. Seahorses are very social fish. Weather you keep 1-2- or 3, not only will they socialize with each other, they will add you to their fish family. There's so much more to a seahorse...I posted this many months ago:
....Seahorses are the most unique fish I have ever had the pleasure of sharing my home with. I do not say keeping, because even though all fish have personalities, there is more to a seahorse. The intelligence that flickers in their eyes as they watch you, and the recognition and trust they form with you...takes them far beyond any fish I have ever kept. Sasha and Saphire have shown me trust by not fleeing when I go into the tank to get their food dish.They simply move around the dish as I take it out to clean, and wait for me to replace it and fill it with fresh mysis.