What causes steamed fish to become mushy?
Assuming that the fish is fresh to begin with...the answer is quite simple, overcooking! Fish cooks far faster than beef, chicken or pork. A great way to steam your fish is to wrap it in parchment paper 
cut and paste the following web address to see how EASY it is to get perfectly steamed fish every time. 
https://www.youtube.com/watch?v=52wYn99cL2I