I’m very sorry to hear that. You did everything right in responding to this crisis and you and Zoe deserved a better outcome. Unfortunately, the gas emboli in her case must have involved vital organs and/or organ systems. That possibility is what makes internal Gas Bubble Syndrome so difficult to resolve.
All my condolences on your loss, Carrie.
Please do try to check the dissolved oxygen levels in your aquarium. If the DO is elevated,so too may the nitrogen levels be higher than normal, and that could indicate a potential problem with gas supersaturation. GBS isn’t contagious at all, of course, but if the underlying conditions that triggered Zoe’s problem (e.g., possible gas supersaturation) are still present in the seahorse tank, it’s possible that other seahorses — especially males that are actively courting are breeding — could be susceptible to other forms of GBS.
Best of luck eradicating the red slime algae and restoring your water quality to normal, Carrie.