My guess is your high PH levels...your plants look similar to mine. The leaves just start to dry out and disintegrate, I was thinking it was because my PH levels were too high (8.4) or because my Nitrates (200 ppm) were too high.

With your Nitrates only being at 30 ppm, it leads me to believe it has more to do with your high PH levels. I also had some healthy looking fish die...from normal, to moving slow, to floaters all within 24hrs. My plants were all doing GREAT until my PH and Nitrates got too high.

How fast did you drop your PH level? That may have taken a toll on your fish.