Liam...first of all, is your CO2 measurement taken before lights "on" or before lights "off"? The prime concern for DIY CO2 is consistency output of CO2...get it between 20-30ppm range during lights "on" period might require frequent bottle resetting. However, once you it down and fixed, you can safely dose 5ml of flourish per 20gallon of tank water 2-3X per week (scale up or down according to tank water volume). Since you bother to measure NO3, why not find your PO4 level? Up it to 0.5-1.0ppm if it is low. For potassium, try to estimate dosing to a level of 20-30ppm using chuck's calculator or 1/4tsp per 20gallon dosing scaling up and down according to tank size.
Note though...nothing works if your CO2 is consistent and high. Good luck.[]
Plant Physiology by Taiz and Zeiger
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