There should be no reason for Excel, I would look closer at your CO2. Get a good ripple going on top and make sure you are circulating the water in the tank. Even to the point of adding a power head for circulation. You want to see plants moving in the current during the photo period. If the tank is packed with plants, even half full, you circulation in the tank will drop off greatly. Without this the CO2 is doing what? rising to the top, where the fish hang, hence there labored breathing. I've personally done the above on several tanks. You will see more pearling and less gasping. Also, the more circulation the more CO2 and ferts can be adjusted.

Also read this, good info
Plant biomass effects over time on Current and Filtration in planted tanks