Hi Matthew! I have a seahorse tank going on 2 years, although only have had the ponies a year and a half. I will share some things I learned the hard way.
1. Build in excellant filtration that is easy to maintenance and does not trap detritus where dangerous bacteria can grow. A sump system works great.
2. Plan for a minimum of 10Xs turnover with areas of high, medium and low flow for the seahorses to choose where they want to hitch. It is a myth that seahorses need only low flow.
3. Plan to maintain a temperature of 74° or below as dangerous bacteria multiply exponentially in temps above that.
4. Plan to keep 1 pair of seahorses per every 30 gallons of aquarium.
5. Plan to buy true captive bred seahorses. Seahorse Source is an excellant place to get very healthy seahorses at a fair price. Also Dan's customer service is the very best.
6. It is a good idea to have a medicine cabinet already stocked before you get seahorses. When they get sick, (and it can happen to any seahorse keeper) treatment should be started immediately for the best chance of success. I think @ seahorsekeeper has a sticky on medicines to have on hand.
I may have forgotten some things so hopefully other keepers will jump in and share as well.