Before making up a nutrient solution, it is essential that the water pH be checked. If the pH exceeds 6,4, acid should be added to the water BEFORE any fertilizer is added.
How much acid to add, depends on the pH and the buffering capacity of the water. Initially add 125ml Nitric Acid per 1000 litres water, gradually adding more until the pH drops below 6,4. Once a rate per 1000 litres is established, a weekly pH check will usually suffice.