Red gills & flashing, no ammonia??

My zebra danio has red gills. Ammonia readings have been 0 ppm (according to the API Freshwater Master Test kit) for as long as I can remember. For the last few days it has hardly taken notice of food, and on the occasion that it does, it immediately spits the food back out again. Just before I moved it into QT it was flashing against the gravel, though I haven’t seen it doing that since moving it. For a while before that it would spend most of its time in a corner by itself.
The tank is the 40g described in my aquarium details.
The fish is not lethargic at all. There are no red streaks or marks anywhere on the fish.
Is it “just” stressed (as in, having good water conditions and more danios would help it), or is this a disease of some sort?
(If it’s relevant, most of the rest of its school died by not eating and getting really skinny. Also, the gourami in this tank used to flash, but it’s been probably a year.)


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The danio is starting to just hang by the side of the tank now.


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What is the water volume of the tank? 40 gallons.
How long has the tank been running? About three years.
Does it have a filter? Yes.
Does it have a heater? Yes.
What is the water temperature? 78 F, though I set the heater for the low 70s.
What is the entire stocking of this tank? (Please list all fish and inverts.)—1 pearl gourami, 5 giant danios, 3 nerite snails, 10 neon tetras, 1 zebra danio. (The other zebras all died a while ago. Most of them had stopped eating and gotten very thin before passing.)

How often do you change the water? Every other week; I just started doing it weekly, though.
How much of the water do you change? At least 50%.
What do you use to treat your water? Seachem Prime.
Do you vacuum the substrate or just the water? I vacuum the substrate.

*Parameters - Very Important
Did you cycle your tank before adding fish? I believe so. In any case, the potentially sick zebra danio was not in the tank at the time.
What do you use to test the water? API Freshwater Master Test kit.
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 30 ppm
pH: 7.5

How often do you feed your fish? A few times a day.
How much do you feed your fish? About two pinches (one directly into the filter output so the neons get some).
What brand of food do you feed your fish? Omega One
Do you feed frozen or freeze-dried foods? Freeze-dried.

Illness & Symptoms
How long have you had this fish? 1 1/2 to 2 years.
How long ago did you first notice these symptoms? Its gills have been red for a while, I think. It’d also been hanging out in a corner by itself for a similar length of time. It started flashing against the gravel a week ago.
In a few words, can you explain the symptoms?
The danio has red gills with no exposure to ammonia. It was struggling to swim to the bottom of the tank, but that issue appears to have abated. It flashed. The fish also was initially uninterested in food once I started feeding it in quarantine. Though it’s eating now, it has some trouble not spitting its food back out again.
Have you started any treatment for the illness? I put it in quarantine after seeing it flashing a week ago. I also gave it a few salt dips, and kept it off food for the first few days in quarantine to see if that’d help its buoyancy.
Was your fish physically ill or injured upon purchase? No.
How has its behavior and appearance changed, if at all? It never to my knowledge flashed before last week. It also wasn’t previously so buoyant that it had to struggle to swim downward.

Explain your emergency situation in detail. (Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)—The danio has had red gills for quite a long time, and has also tended to stay in one upper corner of the tank (only after the deaths of its companions). A little over a week ago, I noticed that it had a hard time moving downwards in the tank, and that whenever it stopped it would immediately float to the surface. A few days after that (exactly a week ago), I saw it flashing against the gravel and immediately moved it into the quarantine tank. I kept it off food for three days before resuming feeding. For the first day or so it wouldn’t eat. Its buoyancy has improved (it doesn’t have to make a visible effort to go up or down). It’s eating again, but it seems to have a hard time getting the food down—it’ll spit it out a few times, or have to open its mouth very wide to swallow. It now alternates between hanging on one side of the tank and (mostly) swimming along the glass walls like it’s chasing its reflection.

No other fish in the tank displays these symptoms.


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The flashing, red gills, and difficulty/unwillingness when eating makes me think gill flukes. (But I'm also currently dealing with an outbreak of gill flukes so that might be just what's on my mind right now.) When was the last time you added new fish or plants to the tank, and what kind of quarantining did you do on them? (Less likely to be from plants, only really happens when the plants were from a tank with fish.) If it was more than a few months it's probably not gill flukes.
Thank you for your response, Deku-Cory!

I added the neon tetras a month or two ago. I usually quarantine for two weeks, but they showed ich at the end of Week 1, so I kept them in longer to treat them with heat and salt. A week after the last spots disappeared, I figured they were safe and added them. However, three of them died during the first 2-3 days of that week, one from obvious dropsy (bloated, pine cone scales). The other two just hung around the top, then turned up attached to the filter in the morning.
I put some vallisneria (that I’d ordered online) in the main tank while the neons were in quarantine. I cleaned it with a brief dip in a mix of 1:5 hydrogen peroxide to water, as I saw recommended somewhere online.

Also, the danio is less frenetic today. It’s hanging around one side of the tank.
Thank you for the really detailed reply!

I wouldn't be surprised if this Danio's problems are related to the Neons. Ich can often be a secondary infection from other problems.

Unfortunately Gill Flukes can only be diagnosed by taking a scraping or a clipping of the gills and looking at it under a microscope. So most people have to just guess based on symptoms. One thing I've noticed when dealing with Gill Flukes is that the fish affected with them do a sort of twitching motion. Not flashing, it's similar to the body motion of swimming, but not quite. It's hard to describe.
If you feel that it is Gill Flukes, it's treated with Praziquantel, usually under the name brand PraziPro.

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