I know nothing about seahorses, but I can assure you that the cyano itself did not harm them. You probably have some kind of other problems. If your water parameters are good, if your tank is cycled, I would look into environmental factors: sprays, room fresheners, aquarium decorations, the quality of your hands when putting them into the tank etc. Again, I know NOTHING about seahorses alone, but if everything you put in the tank dies, I would say it isn't seahorse specific.
i assume its something else and everything you listed im pretty anal about...although that doesnt mean im not missing something