If your 12-gallon nano tank is 20 inches tall, then it has sufficient height for a seahorse setup, but it is much better suited for smaller ponies rather than larger breeds such as Mustangs or Sunbursts (Hippocampus erectus). The 12-gallon aquarium is simply not large enough to safely support 4 Hippocampus erectus seahorses.
So I suggest that you select some of the smaller Shetland pony class of seahorses for your 12-gallon nano tank instead. The "Shetland pony" category include Zulu-lulus or Cape Seahorses (Hippocampus capensis), which are temperate seahorses that require an aquarium chiller to maintain suitable temperatures, or the Black Seapony (H. fuscus), which is a tropical species that would do well at standard aquarium temperatures.
Both these species are small enough to try in 12-gallon aquarium, and they are both considered among the easiest of all seahorses to breed and raise. Like Hippocampus erectus seahorses, H. capensis and H. fuscus seaponies are large enough to eat frozen Mysis as their staple, everyday diet, so you don’t have to worry about providing them with live foods. Please contact me off list ([email protected]) and let me know if you would be interested in keeping either of these species, and I will be happy to send you a species summary that explains everything you need to know about them in considerable detail.
Best wishes with all your fishes, Catherine! Good luck finding the perfect ponies for your 12-gallon nano tank.