how to take care of a goldfish?

It might seem like overkill but one goldfish should be in a 20 gallon aquarium and each additional goldfish should have 10 gallons. That is if you wish to keep your goldfish in good health and it have a long and happy life. They are dirty fish and should have a filter in their tank especially a too small of a container. If (and I am hoping that you do not have a round bowl) you do have a bowl then make sure to not fill it too much past the halfway mark so that there will be as much air surface ration as possible. This is where the carbon dioxide is released and oxygen is taken into the water. Try to think abut the surface of the water as being a straw that you have to breath through. The smaller the straw the harder it is to get enough good air through but if you have a large straw then you can breath easier.

To take care of the goldfish you must check the water quality. The dissolved oxygen, the nitrate, nitrite, ammonia content of the water. The alkalinity and the pH.