My first marine tank set up was for seahorses. I have no trouble with them.
I know that captive bred seahorses are available all over the world and eat FROZEN food. I can not understand where the notion that all seahorses eat live food only came from.
As for tank size it depends which species of horses your are wanting to keep. The dwarves need fairly small tanks. Like 5 gallon or so. And no live rock or live sand or water from the ocean for the little guys. Hydroids will kill dwarf seahorses and their fry. Hydroids will also kill larger horses fry.
As for filtration anything is fine. As long as the flow is not too strong. I started with an underground filter. But I now have a hang on filter.
Breeding the horses is the easy bit. Once they are mature enough, they constantly mate. Raising the ponies is the hard part. They need live baby brine shrimp. Enriched.
I personally know of a lot of people who raise both dwarves and larger horses. Capensis, erectus, reidi, barbouri and procerus are all being bred and raised by people I know.
If you would like any more info you can pm me. Saves people in this forum getting flabbergasted because their misinformed beliefs about seahorses are being proven incorrect.
It really bugs me that someone has asked a question in this forum, a legitimate one at that, and can be answered correctly by the right people yet all they get is a tirade about how hard it is to keep SH and not to do it. So what happens when a person can't get the correct info and goes out and spends a lot of money only to have everything to fail? I know what would happen.... A lot of the members of this forum would turn around and say " I told you so." Whose fault would it really be