I need help please

Im a new grower I kinda hop in the gardering life pretty fast but I do read a lot on growing n setups. I have 3 plants under a 300watt led light in a flood n drain system. I water one time every other day rn They only been in the system for 9 days so I barley started getting the plants nutrients ( floragro ,floramirco, florabloom) and I been growing the plants almost 6weeks now and they been thru a whole lot already and they were doing great for the first 4 or 5 days and then my plants started showing signs of a lil bit of yellow on the tips and I can't figured out how to stop the tips from turning yellow and I changed the reservoir and check the ph multiple times it's between 5.5-6.0, I'm letting the rapid rooter dry out right now to make sure everything dry at the roots before I feed my plants again. Any suggestions please please thank you