That is a good idea, I have one for each of my other tanks that hold 4 7-day containers for a whole month. When I said I pre-mix, I meant pre-mix the dry with water so I can get the dosages accurate, or consistent. I was thinking, for example, I could use the amounts of fertilizer for a 20g tank mixed with say 20mL of water, that should give me 4 5mL doses for a 5g tank. My main concern is if this will keep if I mix a week, or a month at a time. Does anyone do this?