are you groing soil or hydro. if i understand right you are mixing nutes and letting them sit for a few days and using it to water your soil/soilless plants. if that is the case this is a big nono. it says right on most fert bottles to use the ferts after you mix with water and discard the leftovers. saving the leftovers for watering a few days later can cause problems that you don't want to deal with.

if you are growing hydro just ignore my rambling.