To me, your set up and dosing looks perfect. The only think I can see to be concerned about is the direct sunlight. What is your surface agitation like? Could your tank water be low on oxygen? The last thing that I did, that seemed to be the final straw, was to run my co2 24/7. I had to turn it down a little bit, and I also have a small HOB, on a timer, that runs at night to help keep the oxygen levels up at night. Have you tested your Nitrate and Phosphate levels?