I don't use TDS to gage when to change. I just do a 1/3 change every 1 to 3 weeks depending on the tank condition and planting and population vs size of the tank. A single pair in a 10 gal is good for quite a while and 50 fry in a 10 needs changed more often obviously. Bill Shenefelt --------------- See http://www.aka.org/AKA/subkillietalk.html to unsubscribe