Keeping big monster fish in a freshwater aquarium can be an exciting and challenging experience for advanced hobbyists. These fish, which can grow to impressive sizes, can be awe-inspiring and fascinating to watch. However, they require a lot of space, a specialized diet, and careful monitoring of water quality parameters to ensure they remain healthy. Some examples of big monster fish commonly kept in freshwater aquariums include Arowanas, Pacu, and Arapaima. While these fish can be a rewarding addition to a large aquarium, it is important to research their specific care requirements thoroughly before committing to keeping them.