Everything is going wonderfully now.
EC still around 1.7EC
I now raised reservoir to 20L of solution, as they drink a lot now. It takes around 4 days for the reservoir to go down to around 2L of solution left, which I discard.
I added a wavemaker into the bottom of the reservoir, mixing the nutrient solution 4 times a day for 15 minutes each time.
Plants are getting around 300PPFD at 24/0 Cycle