Sorry for the delay in reply, I read the post before but was going to reply later. Then when I had the chance, just forgot.
When adding water to top off the reservoir, normally you'll want to just use plain water. Depending on how long you go in-between
nutrient changes (the whole reservoir), you may want to top it off with a light nutrient solution instead of just plane water. Not at first, but the longer it goes in-between changes, like at the end of the first week or going into the second week.
Also how big the plants and root systems get, the more
nutrients they will use up. So the size of the plants would play a part in deciding to top it off with a light nutrient solution instead of just plain water. Especially in relation to the plant size compared to reservoir size.
Just because the plants use up 5 gallons of water from the reservoir, doesn't mean that they absorbed 5 gallons worth of the nutrients that were in it, and in proportionally even amounts. The plants will absorb the nutrients they need, and leave the rest. But as mentioned not in proportionally even amounts. The elements that they don't use up as fast as others will remain in the solution. That is why although helpful in some ways, EC/TDS PPM meters are not necessary to grow hydroponically. They can give you the relative strength of the nutrient solution, but cant tell you what elements are in it, and/or in what proportions. Over time as the the plants use up the elements they need, and new nutrients are added to the reservoir, the elements that the plants don't take up as fast as others will wind up in very unbalanced amounts (possibly even toxic amounts). Even if you have the the right numbers on the meter. When you change the nutrient solution, you can be sure you are back to a balanced nutrient solution (meter or no meter).