Danio Ill Health

Can you tell us, did you do a fishless cycle with ammonia solution or fish food or a fish-in cycle with these danios?

The reason I ask is that symptoms like you describe are often caused by poor water conditions, and if you did not do a fishless cycle before getting them, they will have been subjected to high levels of ammonia and nitrite - unless you did daily water changes to keep either chemical from getting above 0.25.


Do you know what the levels of ammonia and nitrite are in your tank at the moment?
Raised levels of both can cause a fish to gasp at the surface, as can damage caused by them in the past. Bulging eyes is usually a bacteria infection caused by the water conditions - and a fish stressed from poor water conditions has a lowered imune system which allows infections to get hold easier.
Some fish are more prone to getting sick than others, this could be why just the one fish is sick.


The best thing you can do for now is plenty of water changes. Good, clean water helps cure a lot of fish ailments. If those don't do the trick, you may need to medicate, preferably in a hospital tank if you have one. Myxazin by Waterlife or eSHa 2000 are the meds to look for.