If your running without a meter you could also try using some distilled water with your mix. About half and half, that's if you repeatedly find tox when feeding less than the schedule. I bet your background water is loaded with minerals, they count the same as nutrients when calculating feed levels. For instance, my background TDS is 500 and I never have a total feed over 850, so no matter what if I use my trap water I can only add 350 TDS feed. So I use reverse osmosis water, by that way I get better control over inputs.