Most of what I've read and followed recommends topping off with water only, until you have added enough back in to match the original amount (ie...once you've added 2 gallons of plain water total through topping off a 2 gal rez, refresh the nutes). Alternately, top off with plain and do a swap once a week or so. The way I understand it, just adding more nutes to a partially depleted system can be an issue because you're adding back everything at full strength, but the plant might not be using as much of some components. For example most fertilizers use the three number XX-YY-ZZ type designation. So while your plant might have used 90% of the XX in the mix, and 85% of the YY, it may have only used 10% of the ZZ depending on which phase it's in. So then when you add back in 100% of everything on top of whats left, there's going to be an overabundance of ZZ compared to XX and YY. Particularly if you're going off of TDS/EC to determine when and how much to add, as a meter only tells the total of everything and not how much of each component is present in the mix.