Fishmonger

Try the one in Stockel on the corner of Rue de l'Église and Rue F Vander Elst

Sorry but for me, the fishmongerl on the corner of Rue de l'Église and Rue F Vander Elst is seriously over priced and not great quality of fish.
The closest good fishmonger is: Claes, Rue du Bois de Linthout 5, 1200 Woluwe-Saint-Lambert http://www.lebancdessaveurs.be/

I agree on the overpriced, hence can't comment on the quality. Personally, I stick to Carrefour or Match for fish, and it's pot-luck for catch of the day.