No, your eyes are not deceiving you. There really are marine and freshwater fish swimming around together in the same tank.
I just spotted this unusual tank at Interzoo which contains a rather unconventional mixture of livestock.
As you can see, there is a shoal of very large and high quality freshwater Altum angels swimming among some Giant vallis, but also some marine Yellow tangs, Caulerpa and even an anemone.
How was it done? Post your ideas below and I'll reveal the answer later today!