We visited all three. Vancouver Aquarium is hands down the best of the three. It's no contest really. Far bigger that the other two and very well done.
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It may not be as impressive as other cities' aquariums, but Seattle's facility offers a wonderful introduction to northwestern sea life. The highlight of your visit will most likely be the Window on Washington Waters exhibit, which houses aquatic animals native to the surrounding area in a 120,000-gallon tank.
The region's premier resource for hands-on marine experiences and conservation education, the Seattle Aquarium offers fun, exciting ways to discover more about the amazing Puget Sound and our world's one big ocean.
Both are endorsed by professional reviewers. Overall, Seattle Aquarium ranks significantly better than Woodland Park Zoo. Seattle Aquarium is ranked #7 in Seattle with positive reviews from 8 sources including Travel + Leisure, Frommer's and Michelin Guide.