Aquapod 12 will be plenty for THE dwarf species
H.zosterae, the dwarf seahorse, only grows to 1.5-2". You can keep 1 per gallon and still barely see them because they're so small.
The Aquapod 12 can hold 1 pair of small H.kuda, which generally are available to market at ~3". When they grow to about 6", then you should consider moving them to a min. 29gal. Their full size is typically 6-8", but we've seen monsters!
The Toronto zoo keeps their kuda breeder pairs in 15gal talls.
You should avoid large species such as H.erectus in this size tank.