I usually use coley with a few prawns thrown in because coley is very cheap and the dull colour doesn't show up in a fish pie.
My favourite is to mix white fish with a little(a third of the amount?) smoked cod or haddock - then a few prawns mmaybe.
I make it like this
Make about half a pint to a pint (depending on size of pie)of white sauce. If I have cream or creme fraiche I will add a dollop.Add plenty of parsley or herb of choice/salt/pepper.
Pour over fish in oven proof dish and mix so integrated.
Cover with enough mashed potato to cover, then top with grated cheese or leave plain. Cook in oven or whizz under grill to brown if everything is still hot from preparation.
You can also heat in microwave then brown under the grill