Quote Originally Posted by herbie the love bud
That's a common error in usage of an EC meter. You cannot effectively top off your rez with more nutrients. If your initial formula is 2.6 EC for instance, and into the week you are at 1.4 you cannot mix up additional nutrients to bring it up to 2.6 again. Because it is a formula of many different nutrients and each with different ECs, when the overall EC falls you don't know what the plant drank. You're assuming it drank uniformly every nutrient.

To keep the nutrient EC level throughout the week in a non-falling rez you should just maintain the same water level constantly by adding fresh water. A float valve is ideal.
Oh I understand that, and my trays are all filled daily on a timer with float valves, but I change the nutes every other week, but about twice a week I go thru them and top up with nutes, I feel I give them 'something' in between nute changes. I'm not saying your method is wrong, and I'm not saying mine is right, but I doubt if there's that much to gain using your method, and mine is less work.