In Modrá, by the folk skansen and near the impressive basilica and pilgrimage site of Velehrad, you can visit Živá Voda. It's a cool place with an herb garden, outdoor representations of the different environments in the Czech Republic, a pond you can wade in (complete with frogs and fish), interactive exhibits good for kids, and the most awesome thing: a glass tunnel under the freshwater pond where you can see giant fish from below. The biggest fish there is a Beluga (European Sturgeon) measuring 2 meters! It's apparently the largest freshwater fish. You can also watch a lot of huge carp and other cool fish. Amazing to see them from that close.
It's hard to show the true size, but here's a hint--with one of the smaller fish.