Sirena, there are two schools of thought on the subject, leave it witht he dealer and see how it does or, if it is healthy, bring it home and get it into optimum conditions stat. If your lfs is above the norm and are providing the anemone with proper lighting and water flow and other animals on the system appear in peak health, then you might be fine to live it there. But, so often lfs don't provide perfect conditions, in which case the anemone can deteriorate beyond the point of return.
sfarid, i have two mags in a 50 gallon under 6 t5-ho. I have one in a 40 gallon (I just moved it) right under a 150 watt, 10,000K MH. I have another in a ~50 gallon system directly under a 150 watt 10,000K bulb. I am using Coralvue brand MH bulbs. I've tried Ushio, but the anemones seemed to shrink under them and require a clear, UV absorbing shield. I've had the Coralvues for less than a month, and so far, I like them.
I'd be interested in knowing what other H. mag keepers (who use MH) prefer in terms of brand and Kelvin.
I suspect that the cringing I saw with the Ushio was the result of UV. This is anecdotal, but still somewhat scientific: There is a brand of sunscreen (Blue Lizard) that has a plastic cap that is sensitive to UVA/UVB rays. It turns a darker blue depending on the intensity of the UV. Under the Coralvue, the blue that develops is much lighter compared to the Ushio. Even with another piece of glass besides the MH glass shield, the indicator turns a deep blue. Because glass absorbs the majority of UVB, I hypothesize that it might be UVA that is being released. I'm sure that someone who has more precise instruments than a plastic cap ;-) could zero on on just what's going on with some brands of MH bulbs. It's a little disconcerting to think that UVA might be emitted considering the fact that UVA is implicated in the development of melanoma in humans. Whatever the case, my anemones seem to prefer the lower UV intensity MHs in contrast to the higher. The t-5s will also cause a faint blue on the indicator, slightly less than the Coralvues, but dramatically less than the Ushio--or full daylight sunshine.