If you like liquid nutes, The GH trio is fairly inexpensive and available everywhere. I like dry nutes because I'm cheap and don't want to pay for shipping water. I've had good luck with Jack's and Masterblend. But the nutes all work.
Under LEDs and in an amended soil mix, the only week I give straight RO water is right before chop. Otherwise, I'm either feeding or supplementing with
calmag (through week 3 of flower) or mag sulfur (week four of flower through the second to last week).
I withdraw my "hungry" guess/diagnosis, btw. 18 days probably isn't enough time for plants that size to exhaust the nutes in that much amended soil.