I don’t know how this question arose – as far as I know [tag-tec]desalination[/tag-tec] has no application with regard to the marine aquarium. This process is where the salt content of the seawater is extracted. As we all know, when water in the marine aquarium evaporates, the salt content is left behind.
Having said that, desalination can occur in a marine aquarium, but it is not desired and the occurrence is very rare. What happens is that the aquarist is very diligent in topping up the aquarium water level with fresh water, but the aquarium also suffers from a very bad case of ’salt creep.’ This is where salt encrustation appears on the glass and/or wires, tubes, equipment etc above water level. It can be caused by spray from an airstone etc but sometimes just seems to appear. If the encrustation continues, salt is being lost from the water and over some time the [tag-ice]specific gravity[/tag-ice] (SG) will reduce. It should be noted that salt encrustation deposits should not be pushed back into the aquarium in lumps, particularly in a reef aquarium. This is because if the slowly dissolving lump lands on a coral it can burn and damage the coral.
An SG test should be completed at least once a week as a matter of routine. The test, which involves the use of a [tag-self]hydrometer[/tag-self], is arguably the easiest one the aquarist needs to perform. Routine water changes should also be normal, and small changes in SG can be tackled by fluctuating the SG in the new water mix a little.
Salt loss and fluctuating SG is not a problem that is often faced. There are other concerns that are much more likely to arise.
Come to think of it, there is a desalination application relevant to the marine aquarium. A certain manufacturer, who has an establishment on the shores of the Red Sea, I believe uses desalination to produce the salt. There’ll be a bit more to it than that of course.