20 gallon res question

ok so I just upgraded From a 5 gallon to a 20 gallon reservoir do I really have to completely add new nutrients every 7 to 10 days. Or what is the best way to go about this I just feel like I'm going to be using so much nutrients.

Well i use 15 gallon and in flower week 3-5 my plants are drinking around 5 gallons evey week so I usally just top back off 5 gallons of phed water then around week 2 I change out the res probly could go longer but I don't like to stress or under feed my girls, In veg I went around 3 weeks . But my plants are also over 5 1/2 feet tall so I'm guessing if your plant is small you could extend longer just don't add back nutes only add nutes at res change by doing this I have noticed a night and day change in growth and health. Only thing I would add back is if I see like a ca/mg def. good luck you will love the bigger Rez way easier to maintain