Hi, If you really want to do the dwarfs I would get one of those 7 gallon bow front tanks. Not only do they look really cool, but they are a good size for dwarfs. Even the 5 bow front would be ok for the dwarfs, but I would not go any bigger or smaller than these sizes. The reason I would not go any bigger is that I tried to keep the pygme's in a 25 gallon once and the tank was too big to get the concentration of the baby brine high enough for them not to starve. These guys will not swim after their food....they just hitch their tails around something and wait for it to come to them. They are kind of a lot of work as one must constantly hatch live brine shrimp for them as they will not eat any dead food. A tank full of baby amphipods and copepods is nice for them, but they cannot be kept with hardly any other fish as they will eat them or kill them.
Ideally they could be kept in a well established refugium that had been up for at least 6 months, better a year. In a little tank you need to get some sponge type filter, or at least a regular filter inlet wrapped with a fine sponge type material so as not to suck the horses in or the baby brine up. The problem with the baby brine is that is sould be enriched with something called Selcon which adds some of the missing fatty acids. One needs to add a vitamin supplement somehow also as the brine are short on those. I think there are vitamin supplements that one can add to the water....don't know if they work. The small tank does need to be cycled for about a month even if you use old tank water. I would use some small pieces of live rock and later add some macro algea for them to hitch their tails on. If you could get a hold of some amphipod and copepod cultures and set the tank up for at least 4 months before you put the pygme horses in the tank you may have enough to help to supplement the baby brine that you must hatch for them. There is something called Life A that comes in a bottle of liquid which are brine shrimp eggs without the shell that hatch. You may be able to hatch these directly in the tank as hatching the baby brine got to be a real pain when I kept these guys. I had to have two seperate cultures going at a time. You cannot put the brine shrimp eggs with the shells in the tank as they may choke the horses and make a mess. I am not sure if the expensive Life A decapsulated eggs make as much of a mess, but they say you can put it directly into the tank. Do a searc on line for this as most LFS dont carry it. Didn't find this stuff until after I no longer had the horses. I may try them again with a 7 gallon bow front sometime, but I am going to grow all types of pods in the tank for at least 4 months before I put them in. It is a real pain to keep them fed with brine otherwise. Make sure that you also pay attention to the water quality and do a 20 to 30 percent water change ever two weeks. This is east in a little tank as 30 percent of 7 gallons is only about 2 gallons. Better to even do it more often.....like every 10 days if one does not run a skimmer.....and it's really hard to find a skimmer for a tiny tank.
The larger seahorses are just as difficult, and require at least about a 40 gallon tank. They need a deep tank as they mate in a deep water colum. They only mainly eat live food also, which is a pain. Good luck, Lesley