red spot at tail of diamond tetra

Is it just the one fish?

Could be something as simple as a ruptured blood vessel.

Personally, I'd quarantine the fish and just keep an eye on things.

lauraleellbp said:

Is it just the one fish?

Could be something as simple as a ruptured blood vessel.

Personally, I'd quarantine the fish and just keep an eye on things.

Click to expand...

I've got him as well as some others quarantined. I've had some unexplained deaths so I'm being pretty careful. Some others have dark areas (black-ish) in about the same spot but they don't look alarming like this thing. Hopefully it was some damage my geos caused while thrashing around (they are temporarily in cramped quarters and are thus more aggressive than usual). I will be keeping an eye on that tank though. Is it possibly bacterial?

Treat with antibiotic foods if he eats. It's a blood infection. You see this in goldfish with streaks of blood in the fins.

Yeah, if more than one fish is showing discolorations in the same area, I'd also go for an antibiotic.