what I would have done is mix 20Liters & kept 5 back but you need a 20L container fpr that ... you should be fine just adding back water with aqua vega mate , let the res drop a few liters & then add back half way through the week & that should last till the next res change. Another 10ml of A&B into the 5 L again bro :Sharing One:
soon you will need to make up a top up res after the1st one as she will drink faster & faster now ... keep a check on how much they drink to get an idea of how much top up nutes you will need each week ,but eventually you will be making up an extra 15 liters & the oxy will drink all that before the next res change .