Gday Mate,
Sorry, but how long is a piece of string.
Seriously, its about size of the species, naturally a tank for dwarves is going to be smaller by far than a tank for say Pots (about the biggest)
For the medium species, 25 gall min, large, many say 29, I say 40.
Seahorses are wonderful playfull things, and so, they need room, if they do not have the room, they sit like Buda in a 10' cell, dull, booring, almost like statues most of the time.
The question Id be asking is, what species do I want, and how biga tank can I get them.