All my condolences on your losses. It sounds like you’ve been through a very unfortunate experience — it can indeed be quite traumatic and terribly disheartening to see your seahorses suffering and not be able to do anything to help them.
Since the water quality in your main tank checked out fine at your lab, and the seahorses recovered when transferred to the interim tank only to become distressed again, breathing rapidly and swimming agitatedly at the surface when you transferred your heater to the new tank, I think you may have identified the likely culprit. Stray voltage from a faulty heater can certainly become a serious stressor to fishes in an aquarium. Just how stressful and how dire the consequences depends on the amount of voltage involved.
Low voltage will be a source of chronic, low-level stress to seahorses and other aquarium specimens, which can be very debilitating in the long run. Higher voltages are increasingly stressful to the point where the aquarium inhabitants are actually getting jolted, which can even result in fish trying to leave the water in order to escape this painful stimulus. And, of course, a catastrophic heater failure in which the heating element is exposed to the water can pose a risk of electrocution to both the aquarist and his fishes.
It sounds like you were spared the worst-case scenario in which the heater shorts out and electrical shock becomes a hazard, Kris, but that your heater may have become a significant source of stray voltage. If so, the resulting irritation may be what drove all the little things out of your live rock initially and caused the distress to your seahorses.
In the future, a simple titanium grounding probe will protect your seahorses and other wet pets from stray voltage and should also safeguard them in event of a catastrophic heater failure.
But the best way to protect you and your loved ones from electrical accidents around the fish room is to make sure all the outlets are equipped with Ground Fault Interrupters. And it’s a good idea to make sure all your electrical equipment is plugged into a surge protector as well to further protect your expensive pumps, filters, heaters, etc. from damage. An ounce of prevention…
In short, if you replace that faulty heater, install a titanium grounding probe in your new 75-gallon aquarium, and observe the precautions outlined above, you should be good to go and seahorses should thrive on your new setup as before.
Best of luck with your new 75-gallon system, Kris!