Might be further off to run your res with H2o2 solution right off the bat. Clean your roots up.I do like the lack of o2 idea, your Ph is whacked, 75-76 is the high side for your solution, probly a few things messing you up bro.
I just ran the H2O2 in my res. because of root rot. Check out the before and after pics of what H2O2 can do for your plants in just hours in this thread here:
https://www.rollitup.org/newbie-central/621291-update-first-hydro-plants-pics-2.html
Its towards the bottom of the page. Also if your using airstones, get rid of them. It help mine out a lot. +1 on cleaning the roots. Rinse them in the sink with your hose sprayer, then dip them in phd water a few times after cleaning them, so they get a good rinse in it. While the plants
aren't in the res, or tanks, clean out your res, your hoses, your pumps, and anything else that comes in contact with the plant or res. water by dumping all the water out. Then puts your pumps back in the res, filled with fresh water and H2O2. Run those pumps for a good 30 minutes allowing everything to get a good cleaning. Now empty that water. Next fill the res. with fresh ph'd water and add your nutes and some H2O2. Hook it all back up, and watch it work. In less than 24 hours, you will be surprised. Also dont freak out and dump a shit ton of the H2O2 in. Just use half the dosage at first to see how it works for a day or so. One more thing, how are you getting oxygen to your res.? Airstone? Just pumping in air?
