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 [tag-tec]cover glasses[/tag-tec], 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!
So, an aquarist should never use cover glasses then? Well, they can be used in certain circumstances and with care.
A fish only aquarium has, well, only fish in it, and all the fish need is to be able to see where they are going and for the aquarist to be able to see his/her fish. Fluorescent tubes are sufficient. There are no other worries, apart from monitoring the seawater temperature in warm weather. Fluorescent lights will not create an overheating problem on their own . Fair enough, the aquarist may position the lights for best effect, and choose different spectrum lights for the same reason (as long as they are reasonably natural for the fish). So in a fish only aquarium there is no reason why cover glasses should not be used. (There is no reason why they have to be used, unless there are fish present which have a tendency to jump.) The aquarist must be sure that there is adequate gas exchange to ensure oxygen enrichment of the water. (Protein skimmers help.) A corner can be cut off one end of a cover glass to leave a small area uncovered. Under this a power head can be placed with its output pointing at the surface, this will bring seawater up and increase gas exchange. Another small corner at the front can be cut off to enable easier feeding. The glasses should be kept reasonably clean. A big advantage is that evaporation will be much reduced, making the demand for R/O (reverse osmosis) water less.
A [tag-tec]reef aquarium[/tag-tec] needs more consideration. Hard corals in a reef tank require [tag-tec]aquarium lighting[/tag-tec] of suitable intensity and spectrum to flourish. If a cover glass is used, no matter how clean the cover glass is, the glass will act to an extent as a filter and reduce the light penetration into the seawater. Many (most?) reef aquariums use halide lighting, and there is the problem of heat. If a cover glass is in use, then heat is trapped and there is a danger, particularly in warmer weather, of the seawater overheating. This may not be a problem in cooler weather, but the danger is certainly there in warm weather. For that reason, in my opinion a cover glass is not appropriate on a reef that contains hard corals. The second reason is that the light intensity will be reduced and, though the hard corals may survive, this is not desirable
So that’s easy then, a cover glass is fine on a fish only aquarium but not on a reef? Well, not quite.
Some reefs have only soft corals. The demand for light by soft corals is less than hard corals, generally. So how about a cover glass on a soft coral reef? Provided the aquarium is not too deep (the soft corals about in the top 12″ or so from the seawater surface, though there are corals that will grow under less light, and remembering that soft corals generally have a stalk so the light requiring part is higher up anyway) then a cover glass can be used. The lighting does not have to be intense, so the use of halides is not a necessity. Fluorescent tubes can be used (a bank of actinic and daylight tubes should be employed, as many as the aquarium surface area will accept). This in turn means that the danger of overheating is much less. The question of oxygenation should be addressed as in the fish only aquarium, and the same goes for feeding any fish present. The same advantage with water evaporation applies – less top up water will be needed. In this case, the cover glass must be kept clean to allow maximum light penetration.
Out of interest, I have kept a soft coral reef that has been running approaching 5 years as described above. The corals grow (sometimes too quickly) and the fish are happy and healthy. The aquarium is 2 ft deep, and all the reef is flourishing, top to bottom.
So a cover glass can be advantageous, but needs careful consideration. But then, what doesn’t in this wonderful hobby.