I have 4 giant danios. Today 3 of them are head down, tail up and swimming oddly. Occasionally spiralling but mostly swimming frantically all over the place, instead of their usual gentle cruising. All seem to have a lump behind the anal region and all have red vents. All other fish are fine. I have managed to catch 2 of them (in case they are contagious) but in a large tank with a lot of driftwood and plants, they are hard to catch! They may be sick but they can move fast!

Any ideas on what the problem could be?

No change in anything, no new food, no new additions etc.

Sorry I can't help but I have lost 3 of my 5 diamond tetra to something that is exactly like what you describe. I too would love to know what it is.

Jennifer has suggested the fish might have intestinal inflammation and gas buildup in the intestines is causing the strange swimming. Stop feeding for a few days, add some salt to the water and when you start feeding again, add lots of daphia, brine shrimp and spiruling flake. Add those things for the other fish as well to help prevent it.

If the faeces are abnormal that could help you identify the problem as well.

I have lost 8 diamond tetra in a similar fashion. Didn't notice the red vent and just one at a time a couple of months apart. They must be prone to this. Shame coz I really like them . I will try Jens treatment next time.

Any good news Caryl?

Yes. Danios seem back to normal this morning. Still seems to be a slight swelling but all redness has gone and fish swimming normally, although still frantic. That is more likely to be because they were trapped in a small container though! Poo appears normal. I have released the 2 I managed to catch back into the tank.

Yes. Danios seem back to normal this morning. Still seems to be a slight swelling but all redness has gone and fish swimming normally, although still frantic. That is more likely to be because they were trapped in a small container though! Poo appears normal. I have released the 2 I managed to catch back into the tank.

Judging from mine I'd say gentle cruising is a symptom, frantic swimming is normal.

That sounded like a kind of fishy constipation. I often wonder about how fish process the wheat in their flaked and pellet food - I doubt they would get much chance to develop a tolerance for it in the wild.

It was obviously as a result of food, as they had been fed the night before, but I don't know why they reacted like this as it is the same food they get all the time and have never had a problem with it.