Take lids off the tank. You should be using a skimmer already in there, so that aerates your water. But with lids off the gas exchange is better. Yiu can point your powerheads alittle more towards the surface to break up the water surface. And, yiur PH is always going to go down whn the lights are off, and will go back up when the lights are on. Most run a sump, with its lights on either 24-7 or opposite the DT lights to keep the Ph more stable. If none of this helps, waht is your ALk, (DKh) and MAG?
Also, what do you mean by your salt is reading high in the red? I didn't get that one. ??