Leave em unless you don't want to. 13/18 is covering >70% of the surface. That little bit of shady area at the top of the tank is usually a worthy trade-off for having good intensity at the bottom. That said, if you have the capacity to turn it up, then you could raise the light as high as 15"-18" without spilling light over the edges. And just for completeness, you could go with the 120º lenses (not quite the same as removing the lenses in all cases....they make 120º secondary lenses that might be useful depending on your fixture....ask the mfgr) and drop the fixture to about 5" off the water and get comparable coverage at the top to what you'd get at 18" with the 90º lenses.