John, Ive never kept them but did some research once or twice. Most if not all of them need colder water than our reefs so a chiller is a good idea. Check out http://www.seahorsesource.com/. A lady named Abby runs this place and is really cool to talk with, I highly recommend them. The website also sells and has info on many species of sea horses and their food. HTH atleast a little, let us know if you go for it.
I'm prepping my new nano cube to be one. Thinking a pair of dragon face pipes and either two or four horses... Right now trying to figure out the heat issue. With no heater I stay at 78-80 since the nano-cube has little to no evaporation.
They make nano chillers, depending on size of Nano. They have a small Heat Exchanger unit, like a probe. Never had one but did have a Heat Exchange Chiller on a 26 gal once, it worked ok.
Fan on a controler would work, but being a nano you might need a auto top off