You mentioned that your nitrates were zero when bb, then I would suggest you go back to that. A nitrate of 20 is going to be an irritant to any anemone and it will not look its best. I've been there done that with deep sand bed, zero nitrates for a long while and then a slow creep up. I've had my magnificas since 2001 and they have been through it all with me. The nitrates obviously didn't kill them, but I could tell before I did a water test whether I had detectable levels of nitrate. The anemones were like barometers. When they look their happiest, the nitrates are undetectable. Years ago I decided to get rid of any sand bed, even a shallow bed can cause problems with nitrates, and since then I haven't had nitrate problems. So, to make a long story short, I don't think you're system is ready for an anemone at this point.