Keep in mind the sand is black and does have some "sparkly" bits in it. I think quartz... Anyway, you can kind of see how some are doing fine it seems while others are somewhat dying away... perhaps I'm overreacting, not sure.
Black sand could point out to its volcanic origin (volcanic glass) and could easily leach minerals as it likes to dissolve. Possibly not bad as carbonate sand but not good either. Sparkly bits could be mica but these two dont commonly go together. Quadtz dous not sparkle. Not much anyway. Your problem thus can also be from excess mineralisation in water.