My female black phantom has just balloned up. Her belly has grown very fat just now. I presume it's a female as she has redish bottom fins. I tested water and nothing out of ordinary. This has happened not long after i just fed the fish. I don't think she/he is pregnant as still quite small but has grown ALOT in past few months and she is the one and only Black Phantom.
This fish is with danios and serpe's. A while ago some danio's had white mouth fungus but don't think that should make any difference as they were moved into different tank where the two are still being treated.
Can anyone help please, their in a 29G with weekly 25% water change (was just about to do it when saw fat fish) only meds i have are melafix and rid fungus.
Ph at around 7. 0 Nitrite and very little nitrates. Temp@26.

Any advice welcomed