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Very Small Amount: Water absorbs a certain amount of free oxygen from the air, but this amount is so small as compared with what the plants produce that it is not important. To cover the aquarium with glass does not reduce the oxygen supply. It does, however, prevent a film of dust and oil from collecting on the water and disturbing the established balance, and it prevents rapid water evaporation.
The second most usual light source is the fluorescent tube, which consists of translucent glass filled with a gas that becomes luminous when used as a conductor of electricity, and which produces a very small amount of heat. The glass tubes are furnished in a variety of straight lengths, from 6 inches to 8 feet, and are from % inch to 2 inches in diameter, each size consuming a different amount of current. Ring and other shapes are also manufactured, and various colors are available.
"The tremendous amount... of excavation for such a small pipe is however exceptional; in other Greek water supplies, the pipes vary in diameter from 4 to 6 inches, and the tunnels are so small that we wonder how the diggers could ever have wriggled in and out of the 'rabbit burrows,' as the Romans called them."
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