It's a well-known fact that when transporting fish, new water is infinitely better than old water. It's an OLD MISCONCEPTION that old water is better. If I have any excuse to give my fish all brand new water, I take it, and they seem much happier for it.
The only time this is NOT the case, is when your old water is so full of nitrAtes, that clean water would shock and ultimately kill the fish or cause any number of illnesses to take over.
The only time this is NOT the case, is when your old water is so full of nitrAtes, that clean water would shock and ultimately kill the fish or cause any number of illnesses to take over.