I had a semcirculatus for ten years from a thumb nail size to full adult colour at four and a half inches in a similar sized tank,i had four small tanks.
I wouldn't have a clue who has it now,i swaped it years ago,it got to big,they may have killed it,who knows,i hope not.
I realise that most marine tank inhabitants may end up belly up or in a belly eventualy,whether it be in 1 months time or 15 years time.
Thats why shops are able to sell fish,because they don't allways do well,infact most don't that are sold,no matter what some people do!
If you give it a spot to hide and feed it what it needs,as with my over sized
fish,they can do very well and you should try to give them the best chance at life if possible.
Infact in a small near to no invert tank(that can be called a quarantine tank) you can treat a parrasitic attack far better than in a reef tank.
If the owner looks after the fish the best he can with what he is able to fit or afford,than why not give it a go.
It looks like one of our species of butterflies we see regularly,i realise it is a young angel fish!
keep the curamine or similar handy just in case,small tanks can be a bit of stress to a fish.
Cute fish mate!