First time hydro grower, need help with fox farm fertilizer question...

HI all, i just need a little information on something i dont quite get...I go by foxfarms schedule and i use what they say, i have a 50 gallon res that has about 45 gallons in it. After i mix the ferts my ppm is supposed to be so high, around a 1000, but mine only reads 445. Why is this? Do i keep my ppms in the right range, or my ferts on the schedule fox farm provides? Thank you for your help.