Mandarins are a curious situation...they are either the easiest fish to keep or the most difficult. (Like my other big hobby...orchids...pink lady slippers are the most difficult plants to keep in your garden...unless you have the exact conditions they like, in which they are incredibly easy...). I have had mandarins last for years in my 55 gal tank...no idea how much LR I have. Some die within 24 hours...I had one that must have been retarded...instead of diving into the rocks to hide like a normal mandarin, it went right into the open. When the other fish kicked the crap out of it, its stayed out in the open...it won a Darwin award, to say the least.....But most normally behaved mandarins do fine in my tank...IF they are healthy to begin with. Before buying one (and I am assuming that you have the right conditions) ask the LFS how long they have had it...if more than 2 weeks, pass on it. Then check the back and "shoulders"...if they are even remotely concave, anything less than convex,, pass on it....belly should be full, but unless its a hollow cavity I wouldn't worry that much. Tank size? Well, 55 gal has been fine for me. My other reef is a 30...no success yet, but I've only tried once with w weak spotted mandarin. To be conservative, I'd agree with the general belief that 55 gal, loaded with LR, is the minimum. Take care, Eric