Maybe it's all a marketing scheme?
You know, "Get your beautiful, colourful ZOAS!" which will be, of course, command a higher priced than those old, run-of-the-mill zoos.
Zoos roll off your tongue better, but zoas is "more" correct. Either way, everybody seems to know we're all talking about the same thing (except for non-reefers who wonder how we can afford to buy a zoo, let alone a bunch of zoos).
I'm with Christy. Grammatically I prefer zoa, but I doubt I'll pronounce it anything but zoos.
Anthony