Do you mean red underneath the belly if so it's septicemia, you will need to medicate straight away with an anti internal bacteria med, the swollen belly can the fish maintain it's balance in the water, are scales stuck out like a pinecone.
its my zebra danios that seem to be affected! i have 5 of them! the one with bloated sides is starting to get pinecone thingys lol... but its been like that for weeks now!!! what happens if not treated like?
If its not treated then the other fish will probably get sick as well. How often are you doing water changes?
Bacteria is always present in the water from the fishes waste so you have to keep it under control. If water changes arent carried out often enough the bacteria build up and then the fish get stressed and can get diseases as you are seeing now.
Do you test the water at all for ammonia, nitrite and nitrate? If you dont have the tests then usually pet shops can do this for you. What other fish and size of tank do you have? You could try an anti internal med which kills the bacteria in the water. But the main thing is more frequent water changes and more water each time. Start with a 50% water change and each day do 30% to try and get the water clean.
ok i will have to get water checked as i dont have that kinda kits! but i have been cleaning them and doing a half water change! i have 3 neons.. i did have algae eaters but got rid of them!
I would treat the tank with anti internal bacteria med. once the pinconeing of the scales it's the last stage of dropsy i'm afraid and you will probably lose that fish now.
Don't worry Nemo... I had a danio who died like that too... the poor thing fought it for a week, she was really huge with her scales standing up.
Two of my guppies died from it too...
Bless the little fishy... good luck.