I'm not a lighting expert but I would say with the space you have, perhaps two 48" set centered both front and back of the fixture and then up to four 24" spread out to fill in the empty spots...like two up front side to side right behind the 48" spread out to cover the entire length of the tank and the same with in back side to side in front of the other 48". This can allow for ways to have actinic bulbs on separate switches to allow a dusk and dawn period for the fish. You would put the actinics on about half an hour prior to putting on the regular lights and then half an hour after the regular lights go off for the night. You could even fit a little tiny 1/2 watt LED
light in the center of it all to create a moon light for night time.
You seem to have a nice and different natural aquatic eco system. Being able to replicate natural lighting as any aquatic system may experience naturally allows you to observe some unique natural behaviors from your fish...and may even breed because of it.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.