For this seahorse, I'd suggest doing 50% water changes daily, and, be sure to use heavy aeration with open ended airline not airstones.
For buoyancy issues you will have to evacuate the pouch.
https://fusedjaw.com/diseasehealth/s...ch-evacuation/
After the evacuation you may also flush the pouch with a syringe using new salt water at the same temp and pH the hospital tank is at.
If you have access to Furan II and or tri-sulpha, I would also use that according to the instructions given on the package. It may not work but at this point I prefer to try whatever I can.
For not eating, I'd buy some appropriately sized shrimp, likely freshwater type is all you can get, but usually they live long enough for the seahorse to take and interest and ingest if it can. Keep the shrimp separate and just add one or two at a time.
Lastly I'd recommend doing a LARGE water change with an extreme cleaning of any trapped detritus and uneaten food from the display tank, followed with another LARGE water change a couple of days later. This may help the others from coming down with the same problem.