My one tank when I first started I had the same issues. Redid my whole ro system. It wasn't that. My alk wasn't even at 8. How could that be? I lowered to about 7.4. Better but still some issues. Could it be alk, or is there some other underlined issue? I got on the phone with adam from battle corals. Sent him some pics. Same thing as what I'm seeing. "Alk burn". "Let it fall".. "quit changing water". So for the longest time I actually kept the tank a tad under 7.0. I think the next line down on Salifert was 6.7 by the way I test. It fixed everything over a month or so. Why, I don't know. I've always dosed N and P to keep both stable and noticeably detectable. My doser is set to dose every hour. I matched water change water with hlcl acid. what the heck? Even now if my ph gets lower because of the house being closed up, my alk will creep up to 7.4 I will get those signs. I try to test every 3 days or so, but who am I kidding. I'm busy and have multiple tanks. I'll turn the doser off over night and back to the normal schedule the next day.