I will have to find the thread to give you a better source but from my limited knowledge dwarf seahorses do better in smaller tanks. I think people were saying 4-6 horses per gallon (if you are actually buying H. Zostera). I am under the impression that they will only eat live food and you will have to have lots of baby brine shrimp available. Like all seahorse species they are weak swimmers and require lower flow, and you will also need to keep your filter intakes covered with foam or something to keep them from being sucked into it. Most people will choose to use a sponge filter to keep that risk down. The heater should also be covered or kept somewhere that the seahorses cant burn themselves on it. Also like other seahorses they have a weak digestive system and its suggested that they be given small feedings a few times throughout the day.
Here is some much more detailed information.
http://www.seahorse.org/library/articles/dwarfguide/dwarfguide.shtml