A T5HO fixture is extremely bright for freshwater, it is better suited for reef tanks that require intense lighting for corals. With that light fixture, you will need to use pressurized CO2 to balance. Add in direct sunlight and you've got a lot of light. The plants will happily photosynthesize until they run out of something, then they slow down or stop and algae comes along and takes over because it is less picky. You are probably short on Carbon. Excel may or may not work for you, it's a chemical and not CO2 and as such some plants do not respond well to it.
You have a couple things you can try. One is reducing your light fixture down to a single T8, or possibly a dual T8 (don't know what plants you have). Second you can invest in a pressurized CO2 setup, it's expensive and has a learning curve due to needing to balance your fertilizers (most people who go this route use dry ferts daily). Third is to close the shades, but only during the periods the tank is getting direct
light, likely dawn and dusk. A final option that will help cut back the intensity of your light is a thick cover of floating plants.
The behavior of the Neon Tetras is expected under bright lights, forest fish dislike it, with subdued lighting they'll readily venture up for food.
Give the Excel a try, see how it responds. Some discourage its use as it is just another chemical, and who knows how that effects fish. I've personally never used it.