Depends on the system bro I've done plenty of no flush grows and believe me I was dead against it for a long time until I started using Lucas's formula and recommended addbacks. you would think of course that some of the element salts would build up to toxic levels but it really doesn't come into play as you would thing when you properly manage your nutrient solutions. I will still recommend especially for beginners res changes every two weeks but not before that is only wasting nutrients as well as adding extra waste to the environment.