Happened to me before with long term high nitrates. My drip wasn't working properly and when I tested it it was only dripping .5gph I stead of 4gph usual
Those two points you see near the tail are the hip bones. Something you don't want to see. If your rays are eating they may have too much competition or you are not feeding enough. They are skinny enough that I imagine they could die from complications of nitrate poisoning. I think it's been sugar coated a bit, you need to make sure they are being fed as much as they will eat, if they are eating.