How do I cook frozen fish in microwave?
I frequently cook fish in the microwave, as I'm the only one who eats it in my house. Thaw filet in cold water and place on microwave safe dish.  I turn fish over halfway through cooking.  The kind of fish and the thickness will determine time.  I recommend using partial power or your fish will be rubbery.  I have a 1200 watt microwave and the following is just a guideline.
From completely thawed:

Salmon filet @2" thick - about 2 minutes each side at 70%
White fish or Tilapia filet (these are very thin @ 1/4") 1 minute on each side at 80%.  

Please be aware that this is just from my personal experience.