I'm glad the remain horse looks like it's going to make it. That's great news! I would still consider the possibility of an oxygen deprived tank, as warmer water holds less oxygen. Bacteria is a suspect, of course, but it generally isn't going to kill that quickly and not leave some kind of symptom. Watch the remaining horse closely for discoloration, especially around the snout and tail. I would really try to keep the temperatures down, and see what you can do to increase surface agitation, perhaps adding an airline to the tank. Actually, considering your in FL, is there a risk there was any kind of power outage due to rolling blackouts?
So I wouldn't rule out bacteria, but my gut says there was a situation where the tank was deprived of oxygen, which would affect the seahorses before the other fish. If the tank got warmer than 79 when you weren't home, and there wasn't enough water movement, that would certainly be a problem.
I hope you know I wasn't trying to attack your ability as an aquarist. The recommendations for seahorses aquariums exist because they *can* be delicate and die at the drop of a hat (see this thread). People very frequently try things that go against "common wisdom", and think everything is fine - until it's not. This is true in every aspect of aquarium keeping, from goldfish to the most complicated reef setups. Short term success does not mean continued success. For example, I noticed that you have corals with sweeper tentacles and other stinging corals. One easy to spot scenario is a seahorse getting stung, stressed out, and opening it up to a secondary infection. Because seahorses have an underdeveloped lymphatic system, it then won't be able to fight the infection, opening up the possibility it infects the others due to increase virulence and spore count. Or it may never happen. Most people are not comfortable with those odds.
I don't want to fight over this, but please try to keep your mind open if you have any additional problems. I was there once, thought I knew better than what others were telling me, did everything wrong, had everything die, and it wasn't until much later that I ate my humble pie and finally did it "the right way".