I just switched from hydroton to coco. Not sure how you have fungus gnat problems with coco but not hydroton. Here's my advice.
Stop feeding plain RO water. Even if you pH RO water, it has nothing in it to stabilize the pH. That is not going to help in hydroton. If your intention is to flush, use a low EC/ppm fertilizer, as that will be more effective for removing unwanted salts. Keep your pH at 5.8 exactly.
Seed Buyer is right, don't water during dark time.
When you refer to your nutrient strength, please use EC. It is constant, whereas ppm can be in the 500 scale or 700 scale. An EC feed of .4 to .5 would be fine right now.
From the pictures, and having no idea what nutes you're using, I'd say you have two problems. Overwatering and also your pH is out of whack, which you are confusing with the claw. Your leaves are twisted (usually pH) and have low turgor pressure because they are being watered too much. Take the covers off the hydroton, that is going to make overwatering worse. Get yourself a short cycle timer, and set it to water for 30 seconds every few hours. If you watch your plants like a hawk, you can then figure out a good watering cycle for your environment.
I run 1 burner as well for my veg and flower. I keep it set a the low (350 ppm-the usual amount of co2 in outdoor atmosphere) and the high at 650- 700ppm. Make sure your burners are completely combusting all of the propane, as that can make plants look stressed too.
Are the temps the same in the veg room as the flower room? Are the plants 80-85degrees at the canopy level, or is that what your thermo reads on the wall? That's too hot with too low RH imo, at least up the RH to 65% the first 4 weeks.
Last, but not least, flipping from a 400 MH to a 1K HPS will stress the plant going from different spectrums.
Hope that helps!
