How Long Will My Nutrients Last Once Mixed?

Only mix what you need to use :) Air stones do nothing I have tried using two in my reservoirs they do nothing but oxygenate the water and well im not growing plants in my res so its pointless what you need is a water pump. just need to keep the water moving yes air stones do move the water but no where near as much as needed to stop your nutrients going bad. Keep in mind as soon as you open a bottle of nutrients that bottle will only stay good for around 6 months. this is the reason I use powdered feeds they are much cheaper and you get complete control on when to make the feeds and when to use them also the powder lasts for upto 2-3 years :)