Thank you for the feedback. I haven't started building the canopy yet, and already owned those lights, so it would be very easy to change plans.
Would T5 or T8 provide enough light for aquatic plants at that distance? Would you recommend pulling the existing plants from the tree, enjoying them in my home instead, and replacing them with more high-light species?
Perhaps a third option would be using the great big reflectors that are on the halides but with a different type or intensity of bulb.