In need of plant diagnosis

Hi, everyone I'm new to RIU I have experience in soil, but this is my first hydroponic grow. I've already killed one of my plants. My submersible pump pulled the newly established roots out. Now my question is what are these spots. I've recently switched to distilled water, but one of my older plants are looking worse than ever. My ppm is around 550 and temps are a steady 70/69 in the water with an occasional rise in heat on a hot day. At first I felt it was ppm but I have other plants and a seedling thriving in this environment. Also one of the plants was too close to my light and recieved some burns