Went there when it just opened (December 2004), parts weren’t finished yet (monitors not yet working etc). Still, as all aquariums, fun to visit. Especially the big glass tank with the walk-through tunnel, which has of course sharks in them.
But a bit expensive though – 9€ for a visit (but there’s a coupon on there website, deducts 2€). Better value is the combi-ticket Aquatopia + Antwerp Zoo, which is just across the square.