When you add nutrients to Grow Buddy, there should be an option to enter the quantity like 1 liter/gallon, 500ml, 250ml and every time you feed your plant, it will take away the amount of that nutrient/additive that was used (i.e. 8ml of voodoo juice from a liter of voodoo juice).
The person should also enter in the amount that s/he wishes a note to appear next to that nutrient/additive when it drops below that entered amount.
It should look something like this
VooDoo Juice: 54ml of 250ml (REORDER)
The person should be able to click on REORDER to take them to the site to reorder that nutrient/additive