Pre-made reef tanks are usually built with a lid/top of some kind, yet most of the aquariums I see in pictures are open on top and have the light fixture hanging, clamped or resting on legs. How come?

There are three main reasons why reef aquarium hobbyists choose to go with an open topped tank.

  1. Glass tops block out some of the beneficial light corals and other photosynthetic organisms need.
  2. A lid/top can trap heat, especially when a high-intensity light is used over the reef tank.
  3. Open tops permit better gas exchange which helps prevent the build-up of CO2 and low pH levels.

There are a some advantages to having a lid/top. Evaporation rates will be lower and fish are far less likely to jump out. Some lids with built-in lights are also equipped with fans to combat the heat problem.

Most find the disadvantages of a lid/top exceed the advantages and thus choose an open top. If you have "jumpers," active fish that tend to jump (wrasses, anthias), you can use fluorescent grids (egg crate) to create a lid of sorts that will help keep your fish in the tank but will still permit line to shine in, too.

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