You need a glass (or whatever) cover, unless you keep the water level very low. The number of marble hatchets I have lost from having them jump out during the nigh when I foolishly left the cover glass open by only two inches after feeding is unbelievable. Floating plants will not help unless the surface is completely and thickly covered, prventing the fish from being at the water surface. All species in the genus Carnegiella, which includes the marbled, are very prone to jumping during play, interaction such as breeding, feeding and especially at night when the activity of lower nocturnal fish frighten them. The larger species in Gasteropelecus and Thorachocharax seem to be somewhat less inclined, but will still jump if startled by lower fish during darkness.
I have kept all these species for more than 20 years, and would never consider a non-covered tank. As for the plant/light issue, a plain glass cover that you clean at every weekly water change is not going to reflect much light if the fixture is sitting on the tank frame. My tubes are about an inch above the glass cover.
Byron.
