I recently "set up" a seahorse tank. I did as much research as possible and there will still some surprises. Here are my thoughts on some things I wish I knew in advance:
Keep the tank with a LARGE well established refugium. Feeding seahorses is a messy business, and if your tank cant handle the excess nutrients you are in for a nitrate and phosphate nightmare.
Buy a chiller. I dont care how cool your house is. Your horses will be MUCH happier and healthier in a cooler environment (74 max). This is not a suggestion.....this is a must.
Brace yourself for color changes you may not like. All my horses started off lemon yellow.....they are all grey, black, or splotchy-grey-and-black. They are still beautiful, but they blend with their surroundings, so it doesnt make for a very dynamic tank.
You cant skip a day feeding them (typically), and they are not as easy to feed as other fish. They really arent hard to take care of, but they do require a little more vigilence.
Buy your horses from one source. Dont mix species and try to get them within a pretty close time frame. Horses from different breeeders will carry different pathogens, which could cause BIG problems.
The more horses you can keep together, the happier and more active they will be. They enjoy saftey in numbers, and frankly are easier to feed in a big group. They also spend more time up and about, as opposed to just remaining hitched all day.
Brace yourself for EXTREMELY limited options for tankmates. I dont care how much you say you are going to have a species only tank, boredom (for lack of a better word) will kick in.
I have 9 horses (H. Erectus) in a 110 tall, with a 65gal sump and 120 lbs of live rock. They seem to be flourishing, even growing cirri (a sign of health and low stress). They live with a mandarin goby and a yellow clown goby, and soon a few reef chromis.
When I look at my other tank (I have two) I hate to admit it, but the tank with the tangs, clowns, flame angel and firefish is simply more colorful and more striking. The horses are fascinating but its like the differece between watching PBS and Fox.