For you guys who had suggested "100% water change" and "jar", sorry it is exactly what we should avoid.
Never do 100% water change, because fish do not like sudden change in the environment. When you give the fish 100% new water, everything in it could be changed. PH, water hardness, nitrate level, water temperature. All of which can kill the fish if the change is big enough. Even if they are not killed, they can be in shock. Frequently put fish in shock will certainly shorten their life.
No fish should be put in a jar. Fish should be in fish tanks of ideal size. Anything less than a 5 gallon really is not an ideal place for any fish since it does not hold enough water to provide stable conditions. All those little tiny jars and fish bowls smaller than 5 gallon are nothing but for marketing purposes.
