Yellow leaves ebb n flow

A question from a fellow grower:

Running a ebb n flo table
Ph 5.96
Tds 900
Temp 72
Water temp 68
Using optimum solution
Running my pump one hour after light goes on for 9 minutes every 3 hours it is going to at least half the Rockwool using plugs with clay pellets in a 3’ baskets
Do not feed during night period. But I started getting yellow leavs in the second week of grow not sure what’s going on could it be my nitrogen is to low went and double checked and tds is 600.
Recipe :250 liter reservoir
A-B =400ml
Mystic root=250ml
Mystic grow=250ml
B1 supermax 400ml
Love to hear you’re outlook

I am a soil guy and not very knowledgeable about your grow.
But I do know thata nitrogen deficiency will start at the bottom of your planf and the yellowing will slowly works its way to the top.
But one Orville he other guys here who know more about what you’re doing will better explain.