I bought 4 golden fish 4-5 months back. I kept them in a bowl of water, was daily removing a glass of water and was adding fresh glass of water.
Starting 2-3 months everything was fine. But slowly one of the fish started getting black spots on its outer surface of body and then died. Then one by one rest of the fish also died.
This also happened when I tried keeping them in aquarium which also had air pump for oxygen. Any one who can let me know what was the reason that this happened?