Looks like old die off and maybe getting wet or too humid sitting down in the tray. Just around the bottom. top of whatever your plants are in should be about the top of your tray. The humidity or wet and a 400 could burn them. Since the new growth looks real good I would say trim the old. When an old leaf turns half yellow it is time for it to go.

Please inspect the leaves for bugs just to make sure and don't keep old leaves around. They rot and cause mould and all kind of problems.