You wont find many people on RS that dont do weekly or bi weekly changes, it just makes life easier for everything. I have a 55 with a 20 sump, but i figured the water volume is about 45 in the tank and 10 in the sump cause its really only half full, and i do about 5 gallons a week. Its right around 10%, and it has worked nicely so far for me. And i have the ability to do much more than that if needed thought, you could do water changes, still use the chemi-pure, and then probably stop dosing trace elements depending on what you have in the tank, the salt mix has alot of that stuff right in it.