Just recently, I read into seahorses... I think they're the cutest fish out I plan to get some sometime.. but I'm short on cash right now.
Seahorse tanks can be 10-50 gallons.. well more if you had the cash to stock it.
When doing a seahorse tank, you want to aim for a more vertical tank shape then horizontal. They like to swim up and down.
I would gt a 15-20 gallon tank, depending on how many I want. You could put 3 or 4 seahorses in a 15 gallon.
They're pretty difficult to feed and somewhat invonvient in that they don't live for very long.. so make sure your not biting off too much.
Good Luck,
Galina
ps- you might want to check out seahorse.org .. great msg board.