I think it's better to fill with pure water because nobody can tell how much NPK stays in the mix. If the ppm drops to much maybe you need to change nutes more often, that mean that the plants are sumotori. I don't have that much difference. For instance, 7th week of flo, starting EC at 2.6 now it's 2.14 after 3 days, water level has not changed a lot, so i let it go until sunday, EC will be at around 1.6, still good, there is something to eat anyway...
Fhydro