Fish Market or Seafood market

Trying not to come across as too picky, but even narrowed down to your preference, the Flemish Region is several thousand square km and c. 250km East to West.

Could be helpful to provide more detail:
- do you need to supply a restaurant, or for home use - large, small quantities.
- where you are
- how far you're willing to travel
- good place = cheap or good place = high quality, irrespective of price?

Please understand that in many cases, people post vague questions here, helpful responders try to help with for example "there's a lovely fishmonger's on the seafront in Ostend" - Only for the original poster, having provided limited context, to come back with "why did you bother suggesting that - it's no damn use to me, I live in Hasselt"

Ignoring all of the above - you could try the Vistrap in Ostend.

The whole of the Belgian coast is in the Flemish region so that might be a good place to start.

There is a fish market at Nieuwpoort, www.denieuwpoortsevistrap.be or you can try the Saturday market at de Panne where "Jens" www.jensnv.be/nl/markten.asp have a stall.

There is a fish van on the midi market that is very competative in price but not sure how fresh it is but certainly. Healed than many supermarkets if you're after value for money

By not sure how fresh I mean it looks and tastes like that you buy in any fish department in a shop but I can't vouch it's freshly caught and not
Prefrozen. However we certainly got a good large lives of fresh salmon that they sliced for us although you have to clean the scales at home

The fact is, finding fresh fish in Belgium is really easy, it's everywhere.

For instance: In Brussels:

The fish monger at the Sunday Saint-Gilles market is really good, fresh and well priced. Not talking about the guy close to the church but the guy close to the Maison du Peuple.

The original Mère du Nord/Noordzee place on the Place Sainte-Catherine has really fresh fish.

Poissonerie Dangel
rue Haute, limited selection, super fresh (also traiteur and eat-in)

La Gamba
rue Dethy 5, Saint-Gilles lots of choice, super fresh

Poissonerie La Baleine
Chaussée d'Alsemberg 859 Uccle
Super prices super fresh.

And there are so many more.

Brain freeze: Mer du Nord, not Mère du Nord of course, though you never know...