Excel will boost plant growth, even if emersed.You could, but it would be a waste to use excel instead of using the benefits of emersed growth. Excel provides carbon, which can also be provided via CO2 injection. However, since you are growing it emersed, the plant can just get the CO2 from the air. You don't need that much water, just enough to keep the substrate soaked. Cover the tank with plastic wrap to keep it moist, give the HC micros and macros however you want, and give it time. I have to warn you, it can be very frustrating watching HC float to the top in big chunks after growing it emersed. Start with fine substrate that is heavy.