Tall phicus going yellow! HELP!

[Murray James-Wallace]

Help!  I have 2 phycus (phicus) trees in plastic tubs, each tree is  ~2m
tall.
Since I bought them the leaves are going yellow and curling and then
dropping.  They are in full sun in Perth Western Australia.  At first I
thought I was overwatering them and now water once per week.  The leaves
have a burnt appearance to some of them.  Not all leaves are affected.  The
"veins"are not obvious as per Iron deficiency - could I not be watering them
enough?
thanks for any help offered,
Dr Murray James-Wallace (just not a doctor of plants - that's all!)
jamwal at wantree.com.au