I wonder at what point it'd simply be more cost effective to put in a skylight, and perhaps extended that down to the tank's surface (canopy if you will), and use a reflective mylar or other high reflectivity material.
2 or 3 of these, yeah I can see how it's a close call, however 890 gallon tank? Yikes you'd need quite a few of them buggers!
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Mike
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