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