I think tarragon would be lovely, don't see why not at all.
Are you poaching the fish first? Or do you want to make the sauce just with milk.
With milk: v gently heat some milk (about half a pint or so) with nutmeg and a bayleaf until almost boiling (this bit can be skipped if you are in a hurry and I quite often do!)
Melt a walnut-sized piece of butter in a small saucepan on a medium heat, then stir in one tablespoon of flour, and keep stirring until the flour smells slightly biscuity.
Starting with a small amount of milk, stir it in a bit at a time. To make sure you don't get lumps, stir with a whisk, and keep stirring so it doesn't stick to the bottom. When it's all in, allow to boil until thicker.
If you do have some fish poaching liquid, you can make it half with that, half milk.
This does 2-3 people, I think.