Need some help

[Damageinaz]

First grow Aero Flo system 4-5 weeks plants getting spots and light green stems look spoted. Having problems keeping water under 80 most the day its 73 to 75 PH I let go down to 5 and taking back to 6 PPM around 450 Dutch Master flower A&B with Silic and Potash lights are t5 HO on 18 HR. Water changed last week and slowly to to 450 PPM as I got some burn last week at around 550 PPM. Only been on Nutrients for two week was all good than I burned them after changing water and flush seem not getting something right. Have some brown roots as water temps water cold most the day little warm when I get up but get it back to 73 75 with in few hour working on a chiller