[fkshiu]

ISO is the how sensitive your camera is to light. The higher the ISO, the more sensitive the camera is to light. Generally speaking, lower light requires higher ISO. The downside is that higher ISO settings will cause a lot of graininess in pictures especially with lower-end cameras like point-and-shoots - low light performance is one area where SLRs will blow point-shooters out of the water.

The basic rule is use as low an ISO setting as possible because you will get smoother richer colours.

The upshot is that most point/shooters can only take high quality aquarium pictures with very bright aquarium lighting and have a lot of trouble in low light situations like actinic-only shots.