Did you add some Prime or some kind of water conditioner after the water change? I lost some fish a few years back because the town water was high in chloramine and I didn't use water conditioner. I also noticed you just set up this tank maybe September or October! Stingrays don't do good in new tank setups unless you are really experienced. Not a good fish to start out with especially in a new tank! Just keep up the water changes and use some water conditioner as per directions and hope for the best!