Leaves curling and dried up..Help

i have my plants in a hydroponic setup under 600w hps lights. the room temp is around 90 when the light is on. vortex fan for an exhaust and a duct fan for the intake. co2 setup for every 3 hours for 15 minutes and its my 3rd day into flowering. leaves are curling and turning yellow.
anyone got any advice?

perhaps it could be nute burn, or too much heat.

anyone else got any ideas?

Check the FAQ it has a bunch of answers.