Okay, for starters, these are not goldfish so feeding them goldfish food is not the best. I would say since you have had the fish for 4 years +/- that this is a case of the fish having organ failure most likely due to age. This is a pre-curser to Dropsy where the scales will protrude out making the fish look like a pinecone. The other danio in the picture appears to be a " normal " sized gravid female so the affected one is not just gravid. It's most likely retaining fluid. You would need to consult a Vet to get an internal antibiotic ( ie Kanamycin or Minocycline depending on your water's pH) but to be honest, the cure in this case would probably cost way more than replacing the fish. Dropsy is very difficult to cure and most often, the treatment is unsuccessful.
