Those aquarists who purchased a ready built aquarium will possibly find that glass covers are built in. They will either be ‘lift-out’ or sliding. The aquarium that’s been individually assembled by the aquarist has covers or not – another decision to be made.
If it is a fish only aquarium then there isn’t any lighting consideration to worry about. It is necessary to consider if any of the fish are known to be jumpers. If they are glass covers are usually the answer, though some aquarists use a fine mesh across the top. Where ‘jumpers’ are present, mesh is used in consideration of the oxygen requirements of the livestock as fish, particularly when heavily stocked, have a high demand. Leaving the seawater surface open to the air gives the largest gas exchange area the maximum exchange capability, provided seawater movement is adequate.
With a reef system lighting is a consideration, as most of the corals that are commonly kept on a captive reef need it at the correct intensity and spectrum. Some corals, though light loving, need less than others. Generally speaking, soft corals need less light than hard types. The glass covers need to be kept clean so that they cause minimal reduction in light intensity. Any material, including glass, will reduce light intensity to an extent. In addition, there are strengthening glass straps that run across front to back on many aquariums and another layer of glass above them will cause additional intensity reduction. I do not know what happens to spectrum if anything. Any change caused by float glass may well be minimal.
Consideration has also to be given to the type of lighting system in use. Glass covers do not usually present any problem with fluorescent tubes, but may well do so with metal halide bulbs. Metal halide lighting emits a lot of heat and this is one of the major problems for the aquarist. The emitted heat tends to warm up the seawater, and one way of cooling it down is to blow air across the surface using electric fans. This would be impossible with glass covers, which would allow heat to increase above the seawater surface. So the aquarist would be constantly checking the temperature situation in the ‘lights on’ period and removing the covers when necessary, whereas with an open top fanned air and heat escape is not a problem.
There is an advantage to using glass covers, and that concerns evaporation. Fresh water evaporates from the seawater leaving salt behind, and it is therefore important to maintain the seawater at the correct level, thus keeping the SG (specific gravity) constant. Some aquarists do top-ups manually, others with larger aquariums use automation. If glass covers are in use the rate of evaporation reduces significantly, as a large percentage of the evaporation occurs at the seawater surface. So the amount of RO (reverse osmosis) water required for topping up will be less. However, there is a disadvantage in this as well, and this is that when the lights turn on at the start of a new day there will be heavy condensation in the form of droplets on the underside of the glass. This will also have some impact on the lighting discussed earlier, until the heat from the lighting clears the droplets.
Twice, on two different aquariums, I have carried out an experiment with glass covers. Both aquariums contained a soft coral reef and were lit by an array of fluorescent tubes. For a measured period of six months I put glass covers on and watched for any changes in the corals. I was unable to detect any change at all on any coral either in growth or colour. They all remained in good health. The only noted differences were that evaporation reduced and I had to clean the covers weekly! I do not claim this to be a definitive result, but interesting nevertheless.
I do use the glass covers when I go on holiday, purely because it reduces evaporation and makes Peter’s caretaking job a little easier. When I am at home they are in storage.
It seems to me that the marine aquarist is better off not using covers. First of all, the light that is being paid for is maximized into the aquarium, and very importantly there isn’t any restriction to air reaching the seawater surface, thus as said maximizing gas exchange. Excessive heat can be more easily controlled. If there are any fish ‘jumpers’, then it is a simple matter to make a light wooden framework to fit the top area of the aquarium and stretch a small-hole mesh (not metal) across it. It would be just as easy to remove as glass covers. Finally, there aren’t any glass covers to clean or break.