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.