I change 5 gallons every Sunday on my 55 gallons tank.I have a 13.5 gallon tank that has been running for almost two years and a 40 gallon tank that has been running for just under a year. I got in the habit with my 13.5 gallon tank of changing the water every Thursday because of how the parameters could change so quickly. I also do this every week on my 40 gallon tank. I was curious to see how often everyone changes their tank water. Sometimes I think i can let my 40 gallon go a little longer but i get paranoid because I am on such a routine.