They shrink as they consume themselves, i.e. starvation. IME, T5's would need to be well ventilated and changed at 6month intervals to support magnifica. It would also want to be directly fed appropriate meaty foods every 2-3 days.
Magnifica is quite touchy, most frequently they are dead in weeks. On occassion they hang in there for 9-15 months of slow starvation even when fed frequently. Then there are the specimens that grow like wild fire and get 18-24" across in the first year and live for many years. Have had at least one of all the above situations, in the same system no less. So little rhyme and reason, I refuse to spend money on them anymore.
In the business, most "sebae" anemones are stressed H. crispa. Less frequently H. malu or M. doreensis.
H. crispa will recover well if given a chance, they tend to prefer attaching to rock under the sand, but some will tolerate a rock pile.
For a 55gal mixed environment, BTAs are a far better selection in my opinion.