do you have any other canna nutes MaG ?.
I wouldn't water down mate ,theres no tip burn & there is yellowing so id say underfeeding more like .Just 15ml of aqua vegga sounds quite low too .If you dont have rhizo then id put another 5ml of a&b into each bucket ...PH back to 5.7 & see how it goes ... rhizo or another root stim is pretty important for seedlings in DWC tho mate ,as is the oxygenation of the res (bigger air pump!) ...res temps ect ,its all gotta be spot on for decent yields . an EC meter would let you check for a drop in EC & you would know there underfed but you cant check that so im kinda guessing here

but i use canna & 15ml of vegga into even 10L wont get the EC's you need & im guessing you dont have rhizo in there either so that will make it lower still .
Subbed up & happy to help if I can MaG "AFN smoke out"