PROBLEM TO BE SOLVED: To enable to always dispose a shutter plate disposed at a partition wall in a water tank so that fishes in each of accommodation chambers are not seen each other, and to make the storage and control thereof convenient in an aquarium apparatus in which the inside of one water tank body is divided into a pair of fish accommodation chambers by partitioning with a transparent partition wall to independently accommodate a fighting fish of Betta taeniata or the like. SOLUTION: In an opaque partition wall plate W partitioning the inside of a water tank body V, an opaque shutter plate S having two or more second small holes H2 corresponding to two or more first small holes H1 of the partition wall plate W and overlapping with the partition wall plate W is guided and supported to slide between an opening position A in which the first and the second small holes H1, H2 mutually fit in and a closing position B in which the first and the second small holes H1, H2 do not mutually fit in to be shut of
