The aquarium at Särkänniemi is a solid medium-sized facility with tropical fish, a good shark tunnel, rays, and displays on Finnish freshwater species alongside the saltwater exhibits. It can be visited independently of the amusement park with a separate ticket, which makes it the most accessible part of the Särkänniemi complex for adults without children.
The Finnish freshwater section is particularly relevant if you are visiting during fishing season — seeing the pike, perch, and lake salmon that you may have eaten at lunch displayed in their living form has an instructive quality.