I grow plants in a hydroponic system. It holds 78 gallons of nutrient solution at a 450ppm(parts per million) concentration. Currently each day 7 gallons of 0ppm water are added from a top-off reservoir to replace water removed through transpiration. At the end of each day my solution is reduced to 400ppm and I have to add nutrients directly to my system to return it to 450.
How do I calculate the concentration required in the top-off reservoir to keep my system at a steady 450ppm?