I’m very sorry to hear that the little seahorse with the damaged tail was unable to recover from his injury. Please accept all of my condolences on your loss!
Most likely he died because a secondary infection set in at the site of his tail injury. The most distal portions of the seahorse’s tail are prone to infection because there is poor blood circulation and low oxygen tension at the tip of the tail, so even superficial injuries to the tail often become infected. If the seahorse does not receive prompt treatment with a broad-spectrum antibiotic, the infection can spread to the bloodstream of the seahorse and become systemic, resulting in death from septicemia.
In that event, the chances are very good that the remaining seahorses will remain unaffected since secondary infections are not normally contagious, and I will be very happy to get you started out with the seahorse training program immediately to help assure that the other to seahorses continue to thrive.
Best of luck with the rest of the seahorses, Liz.