Sounds as though I’m getting too personal! I’m not, it is definitely to do with the fish only aquarium or reef aquarium. It is equally relevant to both. Another possible target of such a question is the protein skimmer. That could be involved, but not entirely.What I’m actually on about is the aquarium water surface. The best condition for the water surface is clear and clean. This is because this area is a major point for gas exchange.
A scummy water surface can be seen best by looking upwards at it through the front viewing glass. The scum is usually a mixture of bubbles, very small bits of organic matter or general dirt, and has an overall oily appearance. If it is there it most certainly is not wanted.
What causes this surface pollution? First of all, the seawater may hold an excess of dissolved organics. These organics can be generated by overfeeding, excess fish, lack of water changes etc. The organics tend to accumulate generally in the top ½” or so below the surface (this does not take account of powerful water circulation features, but is correct overall). Organics are attracted to an air water interface which is why this occurs. This is how the protein skimmer functions, by presenting a large air/water interface for the organics to ‘stick’ to. This air/water interface is, of course, the mass of tiny bubbles within the skimmer chamber.
One method of removing the surface pollution is to use strong absorbent paper sheets, which are laid on the surface gently and then gently removed. Much of the pollution can be removed this way. It is not particularly easy, however, and doesn’t treat the cause, so it is likely that the scum will reappear.
So, if scum has appeared on the water surface, the first thing to check is the protein skimmer. Is it functioning correctly? Is it cleaned regularly (scum accumulation in the skimmer neck will impair the rise of foam towards the collection cup)? Has it the capacity to deal with the water volume? As a general guideline, the skimmer should be rated around twice the capacity of the aquarium system net gallonage.
Having checked this, the next check is feeding. Overfeeding can increase organics (and nitrate/phosphate) so the answer is – don’t overfeed.
Water changes should be completed on a routine regular basis. Not only will this reduce organics, it should reduce nitrate/phosphate to an extent and go some way to replacing trace elements.
Many aquarium systems nowadays will not suffer from surface scum. This is partly to do with vigorous water movement, but also because many systems use weirs or other overflows to deliver water to a sump below the display aquarium. These weirs and other overflows allow the water to flow out of the aquarium at the surface, and thus accumulating debris and organics go with it. The water when in the sump is available to a protein skimmer when organics are removed.
A clean water surface is essential. This is because, as already said, it is a major area for gas exchange. It is where oxygen is taken into the water. Oxygen rich aquarium water will deliver, other things being equal, healthy livestock.