A pair of medium-sized seahorses could be housed in a 20g high tank as per the general stocking rules. It is not ideal, but it is fine. Medium-sized seahorses include H. Fuscus ("sea pony"), which can be bought from seahorse source, H. Fisheri ("Hawaiian seahorse"), which are currently available only through Ocean Rider, and H. Capensis ("Knysna Seahorse"), an endangered species currently bred only by Ocean Rider. I suggest you purchase H. Fuscus from seahorse source for three reasons: firstly, Ocean Rider has a spotty record with regards to the quality of their 'horses and customer service, secondly, Ocean Rider's seahorses are far more expensive than Seahorse Source's (~$170 vs. ~$80), and thirdly, Ocean Rider is located in Hawaii, which means that your seahorses will experience far more stress during the shipping process than if you order from seahorse source, which is located in Florida. That said, any of the three species could be healthily housed in a 20g, and both Ocean Rider and Seahorse Source have a long history of success.
Buying captive bred seahorses (wild-caught is nigh impossible to find as far as medium-sized seahorses go, anyways) means that you seahorses will have been trained to eat frozen foods, such as frozen mysis and frozen brine shrimp. Live food, like copepods and gut-loaded brine shrimp can be offered as treats, too, but it is not necessary. As far as frozen mysis, Hikari Mini-Mysis are appropriately sized for medium-sized SH. Difficulty... I have no personal experience with any of these species, but in general, if you do your research, set up (and maintain) an appropriate environment (72-74 degrees F, no stinging corals or aggressive tankmates, etc) and purchase captive-bred seahorses, it should not be particularly difficult to keep happy, healthy 'horses. If you have not done so already, read the articles on seahorse.org about the basics of seahorse care. They are very helpful and informative, and represent the current extent of our knowledge on successful seahorse-keeping. Everything I have said above is information which can be found in those articles. If you are a beginner in the hobby, I suggest that you spend several months (4+) just learning/practicing the basics of maintaining a stable environment. If you have no difficulty and feel confident in your ability to provide a stable environment for you seahorses after that, then go ahead and order them. Patience will pay off in the end.
