The plants could grow faster (if they have enough nutrients and CO2 because 6 H2O + 6 CO2 + Energy = 6 C6H12O6 + 6 O2), if the light is always on. However, like what Bbush said your fish are probably stressed out and that makes them more susceptible to disease.
Also, when you ask "How Can I Improve My Tank?" do you mean fish, aquascaping, or plants?