He's in my 45 gallon pentagonal corner tank, and been doing good too. His spines have doubled in size (atleast) in the six weeks that I've had him. I keep the calcium at about 420ppm, I think that helps.
Yea, I know they are venomous. The guy at the LFS that got it for me said that a couple of years ago he got a spine in his palm. He got it out as best he could. He appearantly didn't get it all, because his hand swelled, and the remaining part of the spine came through the opposite side of his hand a week later.
I had to wait an hour until I could feed my anemone once. He was in the way, and the feeding stick wouldn't reach it. Once things were good in clear, I used my hand.
I like how they aim their spines at things that come close to them.