I think I have Phosphorus (P) deficiency during vegatative growth but I am not sure

Top feed hyrdro system lights are on 250. I have mix of grow cubes and expanding clay balls. I just transplanted these plants 4 days go. I have 10 gallon tank with ph about 6.0 to 6.4 with 5 gallons of water. My starting nutrient mix is root 66 and sugar daddy I put one single does for one gallon of water in with my 5 gallons of water. I started feeding them like twice a day in the last day I have watered them once a day now. The tips are slightly burned on ends I was thinking its a Phosphorus Deficiency. If any one has advice I would very much like to hear it

What are the temps? How close the 250? Hows the roots?, also get rid of the cubes as they reduce O2 in the medium.

Temps are 76 to 79. the 250 is about 16 inches and I have 2 t 5 lights 4 inches from. The cubes no can do and they already mixed in and set in I would have to replant all of them and the cubes actually increase the O2 levels and there mixed with expanding clay balls. So whats the fix for this magcal?