buy them in most bargain stores these days..look like poly bags but are specifically for popping food into for steam then disposal...range sells them too..
it will depend on the bit of fish..for instance a whole trout I give about 90( on full power) seconds if from fresh, if from frozen I do half power for about half the cook time....just have a wee poke and see..not ever long though...
the defrost setting tends to be low power so i think murray is saying cook on defrost for half the time, which defrosts & starts to cook the fish, then on full power for the other half & then test