I wouldn't keep H. erectus or any other species of seahorse (tropical or not) at 78 degrees. In the ocean and in very very very large sterile breeding systems, it works because the bacteria count is dilluted, but in a home system, the bacteria count would get too high and you would most likely end up with a sick or dead seahorse. In the home aquarium, you want to shoot for 72-74 degrees for tropical species of seahorses like erectus, reidi, and kuda, because at temperatures above 74 degrees, the bacteria that seahorses are most sensitive to reproduce quicker and mutate into more aggressive strains. Treating a sick seahorse is a much bigger PITA than most fish, since they tend to go off feeding when they are sick, and they can only go 4 days without food before their digestive systems shut down and they starve, so sick seahorses have a tendency to need you to tube feed them... which is something you will want to avoid by any means necessary.
You also wouldn't want to attach it to your reef tank because seahorses put a heavy bioload on the tank, and will add wastes and nitrates to your reef, which I assume you don't want (it is just adding a predator tank to your reef afterall).
I'd just set up a second system for the SH, they are well worth it.
