Millionaires' Mansions enters the private world of luxury interior design, where prestigious designers compete to work for the wealthiest clients. With access to the houses, the craftsmen and women, and the clients, the series explores how the millionaire lifestyle is created in homes of sumptuous luxury. As the luxury interior design world flourishes, Britain's craftsmen and women are having a creative renaissance. With every client wanting something unique and bespoke, artisans are pushing new boundaries with what can be turned into interior design. This episode meets four of the super-rich's favourite 'makers'. What could scream success more than a 12-foot aquarium full of piranhas? Aquarium Architecture design bespoke aquariums for people who want to make a statement. Russians love predatory fish; Americans want super-sized tanks crammed with fish and coral. The company install a tank for a wealthy Londoner who wants a tank of ugly predatory fish.