Typically, a feed schedule or label instructions are for 1 gallon so if you are filling the reservoir you would multiple by 17. However, if you are using a DWC system (where the roots sit above a reservoir with water) you want to leave air space above the water line between the plants. They need air space to breathe. I usually fill my reservoir half way so if you use a 17 gal you would want to fill about 8-1/2 gallons, which you would need to add whatever the nutrient label says times 8.5 gallons.