Does your aquarium sit in an area that is brightly lit all of the time, or at least during the times when you have the lights turned off? A quick and easy solution to provide your aquarium with darkness is to cover the glass (or acrylic) sides with a black felt cloth. Simply attach a strip of velcro to the aquarium and slap a stick a piece of cloth to it. It’s as simple as that. Keep in mind that you shouldn’t cover the top of the aquarium, as it may block fresh air from contacting the water, driving down dissolved oxygen concentrations and potentially causing some harm to your aquarium’s inhabitants.