Cover glasses of course are those sheets of glass that rest on the strengthening straps at the top of the aquarium. Sometimes they are fitted in plastic holders so they will slide, sometimes they just lay straight on the straps.
The overall practice nowadays is not to use cover glasses, but leave the water surface completely open. This is generally good practice. Firstly, it allows maximum light to penetrate the seawater, which is of particular importance on a hard coral reef aquarium. Second, it allows a large area of open seawater surface for gas exchange to take place. Third, it is one less item to clean!
Read more →