When keeping surface dwelling fish like arowana's, african butterflies, gars etc i always keep the water level two or three inches below the high water mark. I do this for two reasons, one so that i can see the fish without having to sit on the floor to look upwards and two so that the fish dont harm themselves against the cover when they aggressively feed on floating foods. If you are worried about the tank not looking as good with a lower water level try adding one or two tall pieces of bog/drift wood which stick out above the surface and add a few floating plants to make the waters surface more interesting.