we have a seahorse/pipefish forum on TRT. here is the link. i am not sure how much traffic it gets, but i am sure it has been covered in the past. you may have to do a search in the forum to find the posts.
How did you diagnose the edema?
Edema is normally considered to be a failure in the kidneys that causes a swelling due to fluid build up. The seahorse does not float, and may sink to the bottom.
It can be confused with Gas Bubble Disease, which is a gas build up and causes the seahorse to float.
If it is a male, and floating, it may be just the pouch and you have to expel the gases from the pouch.
For treatment, you need acetazolamide which may require a prescription.
Description of treatment is found HERE but you will probably have to join the "org to view it I think.
Rayjay and Geoff in answer to your question, i didn't exactly diagnose the edema, however being a nurse I am familiar with the signs and symptoms in humans. when my seahorse started to swell I became concerned and rang a vet who is studying marines and we discussed the seahorse. Initially I thought he could be pregnant however he doesn't look like the pictures of other pregnant seahorses, and after massaging his pouch twice with no visible air emerging I joined both The Reef Tank and Seashorse.org and posted pictures. Due to the time difference in Oz to the rest of the world I also posted the question here hoping to get a response quickly. I have just finished setting up a hospital tank, acquired and have dissolved the Diamox and am about to put seahorse and diamox into the tank and pray that I am not too late to save him.
Kind regards Anney