Abstract: A dual-purpose cap for a jar-aquarium comprising a topwall having (a) at least one ingress aperture sized to accept free unbroken water-stream passage therethrough for cap-on filling of the jar-aquarium and (b) at least one egress aperture smaller than the ingress aperture and spaced therefrom to facilitate controlled cap-on out-pouring of water, whereby jar-aquarium water may be changed without removing the cap. A series of egress apertures may be grouped together along the edge of the topwall.

Abstract: A multi-chamber selective-exposure aquarium apparatus including: a first portion forming a first aquarium chamber having a first transparent primary wall; a second portion forming a second aquarium chamber having a second transparent primary wall; and a visual barrier device for selective exposure/non-exposure of the first and second transparent primary walls, whereby fish in each of the first and second aquarium chambers are selectively visually exposed/non-exposed to each other's presence. Fish in separate aquarium chambers are preferably male betta fish.