Some ready built aquarium systems are sold complete with cover glasses fitted, often of the sliding type. There could be two or three, perhaps more on a large aquarium. A standard aquarium would need them added. However, when considering a marine system are cover glasses actually required?
They do have their advantages. The first one and hopefully one that wouldn’t normally be required is that the casual flick of cigarette ash or whatever by say an inconsiderate partygoer would not enter the seawater. This would also apply to accidental additions to the seawater even by a careful aquarist, though perhaps unlikely. Protection against these kinds of events could be desirable to some, but are not really essential because the events are unlikely.
Ok, then what of the aquarium inhabitants themselves? Fish for example, some are known to be ‘jumpers’. A cover glass would protect from the danger of a fish leaving the seawater permanently. However, the aquarist could provide a covering net which would serve the same purpose. Also overall the potential for fish jumping is low.
Are there any other reasons for or against cover glasses? Of course there are, there can be a debate about just about anything!
Having cover glasses fitted means consideration of safety from the start, particularly with ones cut and fitted by the aquarist as opposed to the already supplied versions which should already be as safe as possible. The glasses should not fit so tightly that they only just fit, they should be reasonably loose so that removing and placing them is easy. This is helped by gluing with silicone a small handle at the front middle of each glass to give assistance with lifting. A handle can easily be made from two small lengths of rigid plastic glued together at right angles and then glued to the glass. It is also important to ensure that sharp edges round the glass are blunted, this is easily done with emery paper or a sharpening stone which will slightly round the edge.
The major advantage of using cover glasses is the reduction of evaporation. All aquarists know that seawater evaporates (the water content that is, not the salt) and there is a requirement to maintain the level. This is done either manually or with a top up device. If cover glasses are in place then the amount of water required to top up will be much reduced, meaning the required reservoir for topping up will be smaller. Evaporation reduction is the advantage to consider, about the only major advantage.
What of the disadvantages then, there’s obviously going to be some. The first one that comes to mind is that the cover glasses supply another item to routinely clean. They sit above the seawater fairly close to it and are subject to splashes and condensation. With a reef system it is essential to deliver the maximum light and correct spectrum to the corals so the glasses must be clean. After the aquarium night period the glasses are often covered in condensation but this clears after a short period of lights on. Nevertheless there is a slow accumulation of fine debris and/or marking. Cover glasses could also interfere with the dissipation of heat, which is not a good idea when warmer months are with us. Aquarists who use a surface fan to assist cooling would need to remove the cover glasses to increase effectiveness.
Cover glasses and reef systems being mentioned I did an experiment many years ago. I doubt if the experiment would be considered scientific, but at least it was an aquarist checking carefully. For a measured six months I ran my soft coral reef system with cover glasses on (they were properly cleaned throughout the period of course). Then I removed them and after a further six months considered any changes. Apart from the expected increase in evaporation there weren’t any or none that I could see or measure. The corals seemed unaffected one way or the other though soft corals are generally easier than the hard types. This maybe doesn’t suggest anything to anyone but I found it interesting nontheless. The lights in use were fluorescents.
So if there wasn’t any difference in the experiment, why didn’t I put the cover glasses back? Simply because I read an article by a respected marine aquarist and scientist that cover glasses could interfere with the ‘breathing’ of the aquarium. The seawater surface acts as a large air/water interface where oxygen exchange takes place (provided there is sufficient seawater movement). Cover glasses could impair it. Also, to my untrained mind anyway, cover glasses must have some effect on the light passing through either on intensity or spectrum. Finally – maybe I’m lazy, I don’t have to clean them!
There is another advantage cover glasses provide which should be mentioned. When the aquarist goes on holiday the aquarium is either left to automatics or a family member/friend kindly agrees to feed as needed and also do the top ups. With cover glasses evaporation is greatly reduced as already stated, this makes it easier for the kind person helping out and the required reservoir of top up water is also reduced. If an auto top up is in use then again less water is required in the reservoir and/or it is less likely to run out.
So when I’m at home to tend my aquarium cover glasses are not in use. I’ve kept them though for when I’m not here.
Most of the reef aquariums I’ve seen are open topped and that seems the way to go. Fish only systems could have cover glasses if the aquarist is willing to accept the additional cleaning.