This was for a seahorse that wasn't eating. Folks at seahorse.org seemed to think it was the beginnings of a "weak snick" for which freshwater dips are usually the first step before a formalin bath and eventually tube feeding.
After setting up my hospital tank and aerating the water, my horse suddenly started eating like a champ again. Not sure why. Maybe he saw the hospital tank and knew what was coming to him
Either way, he has eaten great at the last 3 feedings and he's already gaining weight back, so I guess this fw
dip is a moot point. But, I will say that I've noticed through all my seahorse research that dips seem pretty common in seahorse keeping for treating bacteria or parasites. Some people even dip their horses as common practice a few times a year if they feed live foods to the tank.
It's still good to know in case I need to do this down the road - thankfully, they are all happy and healthy again.