Biggest mistake I ever made was thinking I was completely able to handle the new task of seahorse keeping as at the time I had a lifetime of aquarium keeping, and, had switched over to reef tanks and fish only tanks a little over 8 years prior. I had approximately the same gallonage that you currently have and then added the seahorse tanks gradually. Unfortunately, seahorse keeping was a WHOLE NEW ballgame. None of my fish EVER had the susceptibility even close to what seahorses have. Probably the worst I came across was my Achilles tang who was the worst of my many tangs, but took me forever to get it settled down and stop the ich infestations on it. Part of the reason for all my tanks was my extreme fondness for tangs and butterflies.
Anyway, back to your problem, if there was NO thrashing and no evidence of any parasites then you should not FWD again. Now it's a case of deciding whether to treat for a bacteria problem or an organ system problem needing Diamox.
IME, law of averages would say to treat for bacterial problem so that means treating with Furan II and tri-sulpha (triple sulpha). Make sure to have a heavy open ended airflow in that hospital tank.
Dose trisulpha by package instructions.
From
http://forum.seahorse.org/index.php?showtopic=42497 find Furan II:
FURAN-2 (immersion) Dosage and Preparation Instructions for a 10g/38L Hospital Tank
Active Ingredient: Nitrofurazone and Furazolidone
Indication: bacterial infection
Disregard package info concerning water changes and duration of treatment. Dose medication daily for
10 days.
Replace the medication in ratio to the amount of water changed daily as needed to control ammonia.
This product is best administered by feeding it to adult live brine shrimp, then in turn, feeding those
animals to the Seahorse. If this is not an option, it may be administered as follows.
DAY 1 of Treatment
• Thoroughly mix one packet of Furan-2 with about 1 cup of marine water.
• Pour the mixture into a high-flow area of the hospital tank.
DAYS 2 – 10 of Treatment
• Perform a 50% water change.
• Thoroughly mix one packet of Furan-2 with about 1 cup of marine water.
• Pour the mixture into a high-flow area of the hospital tank.
Now IME of 25yrs of culturing and selling artemia, ignore the feeding to brine shrimp first unless you have a way to emulsify the Furan II solution.
Artemia ARE filter feeders, however, they filter the appropriate sized particles from the water VIA THEIR APPENDAGES, passing the particles forward to their digestive tract via the forward appendages (legs). While there is SOME solution entering along with the particles, I personally don't find it sufficient to be of any value over the just plain tank treatment.
If you can emulsify it then those particles CAN be taken in and passed to the digestive tract.
In my early years I tried both ways as well as both together and I NEVER found any improvement loading artemia.