I am curious to know what method everyone employs when they do their water change. For ease of conversation, lets say you have a 100 gallon tank and do a 20 gallon water change. Do you:

Option A: Take all 20 gallons out at once and replace with the new 20 gallons.

Option B: Take out a small amount (say 5 gallons) and replace with 5 new gallons. Repeat until new water is gone.

Option C: Something I didn't think of (please explain)

In the past I was an Option A guy but recently I tried Option B. Option A always missed up my levels between my tank and sump. Option B did not give me nearly as many problems.