Yes and no.
The easiest and most straight forward way is to get your 20L of water, add you nutes, then just leave the plants for a week, empty the res, refill and repeat.
However, you can add every day, by using the LUCAS formular, but in my opinion it is more effort and complication than it is worth. It involves measuring the strength of the remaining solution and adding new solution of a certain strength that will mean new + old = the correct reading. I always opt for the new res option, if the res is a little low on water and i'm not quite ready to refill it with solution (time issues for example) it is absolutely fine to just add 5L of ph'ed water to the res for a couple of days.