You collect the water to save money, then spend even more money on loads of chemicals. Is this some kind of cabal ploy? Feels like it. I would uninstall the tank and be happy.
My water butt is working well, collecting the rainfall off the summerhouse, which is at the extreme rear of the garden - a long walk carrying a watering can. She can just draw off what she wants now, straight from the water butt. Reading the discussion on comfrey (thanks), we have just ordered some seed.