cichlid not putting on weight

I picked up a collection of malawis from a specialist fish shop three
weeks ago, so far all are doing well and putting on weight, bar one
which swims with fins often contracted and even though he/she eats is
not putting on as much weight as his companions, two of which are the
same species

This week I am moving all the fish from one tank to another, but was
tempted to hold back this one or put him in an isolation tank to try
and solve the problem

However, my problem is what is likely to be wrong with him - does
anyhone have any suggestions, he is not thin but increasingly the head
is looking out of prorportion

Brian