Zoanthids and mushrooms are your best be to cover a bunch of surface area quickly with really no work. Lots of people with established tanks will often whack you off small bits of zoas as long as you pay then for shipping.
Green star and blue star polyps are another easy choice for covering a lot of area, but be careful, if you dont watch them they can run rampant with growth.
Ricordias are really nice but often can be fairly expensive for the size, and even more expensive to get the super bright ones.
I wouldnt suggest Acans just yet, see how some of the hardy SPS corals do like previously mentioned first. You will be able to sustain them under your current lighting but you will definitaly have to supplement them with target feeding and they will most likely lose a lot of their vibrant colors, which is really the best reason to get them.
I have limited knowledge of the SPS world, but i had good luck with Green Bali Slimer under similar conditions. The guy who sold it to me said it was the hardiest strand he had come into contact with, lasted him 10+ years and through 2 tank crashes. When you get to the point where your willing to try it, if you pay for shipping i will send you a small frag to try out.
Green stick looking one left of middle. (dont mind the algae..all the frags have been cleaned since then
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