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Hello,
I acquired a trio of Robustus corys about 7 months ago. I have not added any additional fish to this tank in that period.
I noticed this morning that one of them has both eyes slightly bulged. (no abdominal swelling)

Tank size 210 gal.
Temp 77
PH 5.5
Ammonia 0
Nitrite 0
Nitrate 10<

No changes to routines in water changes
20% per week

Other tankmates:
Brochis splendens
Hoplo cats
Megalechis cats
Hassar cats
Hemidoras cats
Ossancora cats
Centromochulus cats
No other fish have any symptoms of distress.

I would say this came on overnight. I was observing these fish quite a bit the day before and feel I would have noticed it.

It sounds like popeye to me so probably separate and treat but I'm confused honestly about it getting popeye with stable water conditions and no new fish. Generally speaking, the only time I ever have health issues with fish is a period of time shortly after acquiring them or after adding new fish to their tank. I'm very consistent with maintenance. Just seemed weird. Any thoughts on this?

I hate treating diseases; I always feel like I'm guessing.

When one fish gets sick seemingly randomly, I presume the tank conditions risk affecting other fish too. So I treat the whole tank, not just one sick fish.

If I were you, I'd do a 50% water change, wait several hours, then remove all fish, stir sand or gravel to release any gases, do a 90% water change, wait an hour or two, then replace fish, and pray.

Good luck, Eric

Strange, dead in less than a day. No other fish showing any symptoms still.