Do fish actually sleep?
There is no doubt that they undergo periods of inactivity, but they still have to be wary, even after dark. This is the time when nocturnal hunters such as certain catfish start to become active, seeking to catch the unwary. The appearance of fish also alters after dark, with brightly-coloured individuals often becoming much paler at this stage.
This change in coloration can occur quite rapidly - within minutes of all the lights being switched off. It is quite normal and not a sign of ill-health in most cases, as their normal coloration will soon reappear. Some marine fish undergo a very dramatic change in colour at this stage.

Do not position bright lights close to the aquarium therefore, because if you switch these on when coming into the room after dark, this will upset the fish, causing them to dart around wildly for a time. It is also not a good idea to turn on the aquarium lights immediately when entering a dark room. Rely on the main room lights at first, and wait for a few minutes, to allow the fish to adapt to this change in their environment.

Many aquarium fish do not like bright lights in any event, inhabiting fairly dark stretches of water. Suitable retreats, such as artificial caves and a well-planted area around the sides and back of the tank will help to give nervous fish of this type a greater sense of security.