imo easiest seahorses to keep are dwarfs. here is my suggestion for them.
1st get a 1.5 gal nano from walmart or petco. like this one.
2nd dont use the filter that comes with it, just the airline and get a sponge filter for it.
3rd add a thin layer of sand and water from your main tank. add some macros from your sump.
4th get a small cleaner crew, 1 hermit maybe 2-4 snails all must be small under 1".
now dwarf seahorses only eat live food, you have a few options here
A. Hatch bbs daily and feed to the horses.
B. just add bs eggs to the horse tank the airline in the tank will keep the bs eggs suspended and they will hatch in 18-30hrs.
C. hatch your bbs in the hatchery daily but dont take all the bbs out when you feed, but do add more eggs to the hatchery, keep the water tinted green with algea paste. empty the hatchery every week for sanitary purposes. now the reason behind this way is that the horses will get a varied size of food when they eat.
since you will be feeding live food there is no need to filter aggressivly.
also keep the water in the horse tank barely tinted green to keep any bbs that dont get eaten right away to survive for a later snack.
this has all been based in my experiance with the dwarfs i started with 6 adults and ended up with almost 30 horse only 6months later.