for reg size horses, you need a 29 gal or bigger. 29 will house one pair. they should be kept species only. There are few fish that can be housed with them. You can keep most soft corals. nothing that stings.
you will need sand, base rock or live rock with no hitchhickers.
filter, either a canister or sump. I like a canister with a spray bar. small power head. everyone has a differant way of doing things.
you want low to med flow with no dead spots.
either macros or plastic plants for hitches. feeding dish
tanks should be cycled and running for at least 3 to 6 mo before adding horses.
Temp should be kept between 72 and 78 degrees. but do better more towards the 72 degrees mark. this can be done bby adding a fan to the top on the tank.
Lots of research on thier care and feeding.