Zebra Danio Flashing and Bouyancy Issues

Liv2011

Whenever my zebra danio stops swimming, it immediately goes up to the surface, and it as to make an effort to swim downwards. Three days ago it also started flashing against the gravel. Its gills have also looked very red for a while, even though the tank is well established (it’s been running for two or three years). Its back is arched, and its belly has been pretty large for a while.
I moved it into quarantine as soon as I saw it flashing. I’ve been keeping it off food in case the buoyancy thing is because of constipation, and I’ve given it several brief salt dips (in case the flashing was due to external parasites, though none are visible).
The main tank is a 40g breeder.
Temp: 78 F. (I’m aware that danios like lower temperatures. The heater just interpreted my setting of low-70s to mean 78. I have the quarantine in the low 70s.)
pH: 7.6 steadily
Ammonia and nitrite are both 0.
Nitrate was 30 ppm when I checked after moving the danio. (I did a >50% water change after seeing that number.)
It may be relevant that this danio is the only survivor of its school and has been alone for a while. I ordered more from a LFS but it’s going to be a few weeks. Could the stress of being alone be the cause of these issues?
What’s the cause of the problem? What should I do?
 

JenC

Red gills can be an indication gill flukes when water parameters are good.

You said he's the last of his school. How many were they, how long ago, and how did the others die?

Do you QT new fish and do you administer any meds during QT?

How big is the tank he was in and what else is in it?
 

Liv2011

There were seven of them. It’s been several months at least, I think. Most of them were large like this one, then one-by-one they got really skinny and died. (Internal parasites?) One of them was bloated for a while, with its spine pretty badly curved to the point where its swimming was impeded.
I quarantine all new fish. I usually dose Melafix for a few days during QT as recommended on the bottle.
The tank is a forty-gallon. The other fish are 5 giant danios, 10 neon tetras (new; they had ich while in quarantine but I treated them with raised temperature and saltwater dips, and all spots disappeared), 3 nerite snails, and a pearl gourami.
 

Liv2011

Update: the danio isn’t so buoyant now. It doesn’t have to struggle to get to the bottom of the tank.
I looked up gill flukes. That would explain the red gills and the flashing, but there’s no excess mucus anywhere on the fish from what I can tell, and it’s not lethargic at all.
None of the other fish are having the gill issues (that I can see, anyway).
 

Liv2011

Anyone?
The danio is showing no interest in food today.
 

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