I was hoping my first post on this forum would be in the brackish forum, not the hospital, but here goes:
If you read my thread on the other forum, my first Figure 8 puffer that I purchased last Monday died one day later under mysterious circumstances. I did a large water change, leaving the water basically fresh (slightly higher salinity than 1.000, but not even 1.001). My new puffer (bought Thursday) came from a freshwater tank this time. I acclimated him over an hour, released him, and he slowly overcame his stress and began prancing around like he owned the tank (which he does).
Today when I left for work, he was fine; ate a couple bloodworms and cruised around. However, when I got home 8 hours later, he was laying on the sand not really moving. After some provocation, he started moving around, mostly going up-and-down the glass in the corner. Then I noticed a pinkish thing trailing from his left gill:

He's been showing labored breathing and will either sit on the bottom or swim around at the surface. I added an airstone in the hopes that the aeration would help.
Water parameters are:
Ammonia: 0
Nitrite: <0.1ppm (Nitrite spike, despite the addition of another ounce of Biospira and lots of seeded media; did a water change yesterday to take it down)
Nitrate: <5ppm
I'm not sure whether this *thing* is some type of nematode, gill flukes, or just a mucosal secretion from the gills, possibly due to nitrite. In the case of nitrite and the rest of the possibilities, I did a large water change to the tank with some matured, higher-salinity water, bumping up the tank's salinity to about 1.002.
Any help with this would be much appreciated. Thanks.