the 6' would be great, but 18" just isn't enough width. especially if you're going to put live rock in there... you need a lot of open sandy area and a very stable rock structure. a collapse would prove fatal if a big rock fell on top of the ray. I would say minimum 36" maybe 24" if you really plan the sandy areas well. preferably a lot more honestly.
you'd also have to consider what kind of ray you're lookin at keeping. that' s pretty much all i can say about rays. don't know much about them.
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