I'm interested to know any other growers using LED's and a water chillers how low they get their water temps without adverse effects. It def seems like a root problem because the plants seemed locked out. if 70-72f temps are okay for the res then even better right now 76-78
The table floods day and night every hour and a half
Remember the temp of your water is directly related to it's DO saturation. I don't do flood and drain, only RDWC, and I'm running without air pumps/stones - I simply rely on the waterfall effect and the water temp. - which I keep at about 68-70, then in the last few weeks of flower I gradually start dropping that temp until it hits about 62 with 24-48 hours to go, which shocks the plants.
Anyways, at least as far as RDWC goes, 76-78 water temps would be a nightmare - I'm not sure if the requirements are the same for your setup though.
PS, like you I'm also running HLG lights - I have the 550V2 in a 4x4 and it easily rivals my old 1000w HID. Just keep in mind with LEDs that they don't throw out useless parts of the light sprectrum, as HIDs do. With an HID, this extra white light gets absorbed by the leaf and increases the leaf surface temperature. With an LED, you don't have that, so it is necessary to run your ambient air conditions higher for optimal leaf surface temp. (somewhere around 86F for LEDs, and 77F for Hid). This, of course, is if you are trying to optimize everything.