How smart are fish ?

I have a problem... in my huge wisom I have covered a side wall
of the fishtank with a mirror-foil. Just to cover the unpleasant view
behind of the tank... and to get some more light reflected back.
Today I let the fish in this new tank...

A pair of maroon clowns and hepatus tang get crazy seeing their
own reflection in the tank wall.

Now my title question: will they realize this is just a mirror
soon or they will continue to fight the other fish in the mirror?
How smart are they ?

It is difficult to take the foil off since it is in a tight space
between the tank and room wall... First day I turned off the lights
sooner and wonder what will the group advice be...
Should I go trought the trouble of removing the foil or the fish
will get used to this reflection soon ? If so, how soon will it be ?