Water Surface Scum

There are one or two things that can happen to an aquarium once it is running. The first, and most desired, is that it will be a beautiful and successful reef aquarium or fish only aquarium. There isn’t any reason why it shouldn’t be if research and design have been adequate.

High seawater quality is a requirement for success. The aquarist will spend time doing tests to confirm that the standard is being met. If it isn’t, then corals may well not extend fully, and fish, particularly the more sensitive types, will signal their displeasure by being less active.

The aquarist faced with a downturn in the quality of the aquarium display will sometimes be puzzled. The first and correct action is to carry out a full array of tests, including specific gravity (SG), pH, nitrate, ammonia and nitrite. The reef aquarium needs more if all parameters are to be checked for normality.

There is one parameter that is not often tested for, and that is oxygen. A carefully designed aquarium should never have a problem with this, but it can occur. A reduction in oxygen could be serious, obviously because the livestock require it, including the very important bio-filtration, and also because a lack could cause a pH fall (this is one indicator in overall testing).

Gas exchange, when oxygen is taken into the aquarium seawater, occurs at air/water interfaces. It follows that gas exchange is very important. One of the major areas, if not the major area, for gas exchange is the water surface in the aquarium. With adequate water movement oxygen is being replenished continually.

If the seawater develops a surface scum, then interference with gas exchange will occur. This surface scum can be made up of detritus and dissolved organic matter (DOM). How can DOM occur at the surface when a highly efficient protein skimmer is running? The skimmer can only deal with DOM that passes through its bubble chamber. The skimmer works because part of the organic molecule is attracted to air. There is a large air/water interface at the surface, so DOM ends up there and not in the skimmer.

This situation must be rectified and it is generally easy to do so. Having first made sure that seawater circulation is adequate, the aquarist can create a weir. This is where seawater is forced to overflow at the surface down to a sump, or trickle filter etc. The surface scum will flow over the weir with the seawater, get mixed in and become generally available to the skimmer. Many reef systems employ a weir or two. In systems where a weir cannot be employed because of the general construction of the system, such as their not being a sump present, then consideration to a sump could be given. There are advantages to using a sump. Where the aquarist cannot employ a sump or just doesn’t want one, then a canister filter could be the answer. The filtration employed will be purely mechanical. There are attachments available from one or two manufacturers that take seawater, or some of it, to the filter from the surface. A flat opening is situated at the surface where seawater can exit to the canister, taking any scum with it. The detritus that could be accumulating at the surface will be removed, and the DOM will be mixed into the seawater and become more available to the protein skimmer.

Even if there isn’t a noticeable reduction in the vitality of corals and/or fish, as part of ongoing awareness an occasional glance at the seawater surface, particularly from below, should be made to ensure that it is clear.

With modern aquarium systems the likelihood of surface scum is small, but the aquarist should be aware of the possibility. Keeping all aspects of aquarium functionality at optimum helps ensure the health and long life of our livestock.

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