There is a lot to figure out in determining the proper size tank for seahorses. In caring for fry or juvenile ponies, you want a smallish tank so you can concentrate food. This is also true for dwarf seahorses.

When housing the larger species, you want to ensure that the tank height is at least 3X as tall as the mature fish (with tail fully extended). You also need to determine how many fish you will keep. My 26G is perfect for two pair of Reidi. If I were to go with 3 or 4 pair, I'd upgrade to a 36 bow (I like bowfront tanks for slow water tanks).

In a 45G, you could house 3 or 4 pair as well. Some opt to have a few pair or more. Seahorses do NOT fill a tank with color and motion like you'd see in a reeftank. Having a few pair gives you more of a sense of a truly stocked tank.

Dave