Excellent question. I top off with fresh solution if the EC isn't too high in the reservoir. That might not be the best or most cost effective way, but it works for me. If the EC is high, which is often the case because more water goes than nutrient in my systems, I just top off with rain water.

Just some advice here. . . PPM is not a consistent term and has led to confusion many times. Books are terrible about this, too. Use the EC data and there is no chance of confusion. Personally, I think that PPM should be removed from all meters, nute instructions, and books. PPM isn't measured, so why use it? You'll notice that just about half the time someone mentions their PPM's someone else has to ask what scale they are using. The two scales are different interpretations of the same EC data. I know that wasn't the point to the question, but I thought I'd mention it anyway.