Looking good so far! I would consider ditching the heater. Seahorses like colder water, so unless you're getting colder than 68* you won't need it. I bet you're going to have more trouble keeping the tank cool enough than warm enough. If you decide to keep it in there, you'll want to cover it with something so the seahorses can't hitch to it and burn themselves. There's this stuff at crafting stores that's plastic mesh and you can make a cage of sorts to go around the heater.
Do you have any other plans for the tank? Are you thinking of adding macros and setting up a lagoon type tank, or more of the typical mixed reef look?