I have always followed the concept of topping off with ph'd rowater only, but I read another opinion about this and for my current grow I am topping off with ph'd rowater w/3-part/or nutes only. No additives; So far they are growing tremendously, without problems...I like to experiment, so this works for me. It might be the method that you prefer, but if not...I have always had success by just topping up with ph'd rowater...goodluck.lw