20-20-20 is NOT an appropriate cannabis fert. Can you use it? Maybe. Will you have problems? Likely!!!

Pool pH adjusters are a different set of chemicals and you really owe it to yourself to pick up plant pH down when you go look at cannabis-specifice ferts (a good veg fert!). In the meantime, it is safer to use cider vinegar- just a little goes a long way- to drop your pH, but TEST your solution before using it.