i would drain as much out of the system as possible,just to be on the safe side. A general guideline for TDS levels : seedlings = 50-150 ppm; unrooted clones = 100-350 ppm; small plants = 400-800 ppm; large plants = 900-1800 ppm; last week of flowering = taper off to plain water. remember u need to subtract the ppm's of your tap water from your nutrients.if your bottle ses 600 and u have 200 to start with.your overal number is now 800 ppm and not the 600 most people think is there.