I've used boiling to remove flesh from a bear head before.
The process isn't really all that different from throwing a chicken in a soup pot. Eventually the meat falls off the bones.
Do need to degrease it though, borax works okay for this, and as 0live said, don't use bleach.
Even on large animal bones, the bleach keeps reacting with them, and they get a powdery chalky texture. I imagine a smaller critter would just corrode away completely.
I still think looking to preserve the entire carcass for something that small is the easiest way to go. Even messing with a mouse or rat skeleton is pretty tedious and difficult, and they are much larger, with fewer, bigger bones then a fish.
You can get epoxy resin in most craft stores, or at hardware stores (they use it to make counter/bar/table tops with clippings, coins, trinkets, etc. embedded in them.)
The stuff in craft stores specifically designed for making paperweights, etc. is probably better for casting.