I would definitely stear away from the Idea of putting lights on the sides of the tank. It is definitely possible and shouldn't kill anything. Aesthetically speaking it won't look good, cause algae growth, plants will not grow straight up. Plus putting the light in the back facing forwards, won't that blind you looking into the tank?
Are you having slow growth?
What kind of tank are you running, what size?
What kind of Lighting?
What fertilizers are you using?
What kind of Filtration/media are you using?