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A taotronics fixture will work, but you best get the dimmable one. I think full strength will be too potent for 12" of water unless you hung it pretty high up.
If you think the spread from the one you have will cover 1/3 of a 20 long it may work for montis, but I wouldn't trust acropora would remain colorful for very long under them. No personal experience with them however. Since you have one now, you should do that research before your purchase.