First time growing and the leaves are turning fronm dark green to super ligth green then yellow

Can anyone help me whit this … On how to fix
I have them in hydroponics ph. Is at 6.0. I have 12 gallons of water. I have 2.5tsp per gallon of dyna grow and i have a bubbler… the water is in rotation the temp is at 76f …so i dont know what it could be

Looks like a nitrogen def