Okay, so far so good. It sounds like you are doing a fine job with the treatments.
Yes, if possible, I would like to see another photograph of Tigger that shows his dorsal fin clearly so that I can get a better idea of how he is progressing. If the FormaGreen is effective, I would hope to see the white band cleared up by the seventh day of the treatment regimen.
If the ammonia level is at 0.3 ppm right now, it is only going to continue to rise and you need to continue the treatment for several more days, so this would probably be a good time to perform a complete water change. Prepare the water before hand so that it is adjusted to the same specific gravity and pH as the treatment tank, or use more water from the main tank for the water change, and transfer Tigger to a separate container for safekeeping while you perform the water change.
If you are using a 20-gallon aquarium for your hospital tank, Claire, it’s perfectly acceptable to refill it with only 10-gallons of clean saltwater providing the lower water level does not disable any filtration you may be using on the treatment tank for water movement. If you only refill it with 10 gallons, when you redose the tank with FormaGreen, just be sure to add the appropriate amount of the medication for 10 gallons rather than a 20-gallon aquarium.
If the newly mixed saltwater (or clean water from your main tank) is not the same temperature as your treatment tank, float Tigger in a plastic bag for 10 or 15 minutes so that he can adjust to the change in temperature gradually before you release him back in the treatment tank.
If Tigger is carrying a heavy load of intestinal flagellates or other internal parasites, that could be a factor in why he has developed an infection of his dorsal fin. Anything that debilitates the seahorse can leave it vulnerable to opportunistic infections. A combination of metronidazole and praziquantel should be effective in eliminating such internal parasites, but I would not treat Tigger until he has completed the entire regimen of FormaGreen and you have removed all of the medication.
These antiparasitic medications can be used directly in the main tank, so you may want to wait until Tigger’s dorsal fin problem has cleared up and he has been transferred back to your display tank with the other seahorses before you use the metronidazole/praziquantel, Claire. That would assure that the rest of your seahorses are also treated for intestinal parasites, which is a good precaution since the other seahorses may also have been exposed to the internal parasites via fecal contamination of frozen Mysis they ingested from the bottom of the tank.
Best of luck resolving Tigger’s health problems and getting him back to normal again, Claire.