Not all marine aquarium systems use sand. These are said to be ‘bare-bottom‘, and the aquarists have decided to forego a sand bed in the interests of cleanliness. When the aquarium is new, the bare bottom does look unfinished, but in a little time if the seawater conditions are correct the bottom is covered in different algae, and this can look very attractive. Note this is not the often persistent nuisance algae, but decorative types.
I would hazard a guess that the majority of systems have sand in. This can be either decorative sand or a deep sand bed (DSB) or plenum (a raised DSB).
The sand used on a DSB is of fine particles, and dirt that reaches its surface has a tendency to sit there. Eventually it is utilised by the tiny life forms that inhabit the area. Some may penetrate a little way but not a lot and not very far. These beds are around 4″ deep or more.
If the sand is used for decorative purposes only it is placed in the display aquarium and covers the base. The sand is usually coarse and makes a very attractive addition to the aquarium. These beds are often up to 1″ deep.
The reason the sand becomes dirty is mainly because of the size of the grains. In the DSB as stated it is fine, perhaps it could be described as very fine. If a fairly strong water current hits it a cloud can be raised. Though problems can arise with a DSB, with proper construction and sufficient life it is not usually dirt penetration.
The coarse sand of the decorative sand bed is a different story. Fish faeces, bits of food, dead algae and other general rubbish can fall to the bed. It breaks down and falls between the grains. The grains are coarse enough to allow tiny dirt particles to follow the ’channels’ formed by the spaces between grains. Enough of this dirt can cause the sand to become clogged. This was a major problem with the under gravel filter of yesteryear, which pulled water down through coarse sand and the dirt with it.
With a decorative bed the way to prevent this is to stir the sand frequently with a clean stick. The stirring can be reasonably vigorous and is best done at the time a routine water change is performed. This allows the aquarist to stir the sand and follow the stirring stick with the intake of the water hose, removing dirt. There isn’t any need to do all the sand bed at once, just a proportion at each change. The aquarist can become very expert at this job.
A DSB should never be stirred or the disturbance can spoil the filtration capability of the bed. As said, a well made DSB will possibly look scruffy but should never become really dirty.
It is important to stir a decorative sand bed routinely as described as it prevents the bed from becoming really dirty, which would make the cleaning job much more difficult. If the bed is really very dirty, it would probably be best to siphon out a proportion of the bed at each water change, thoroughly rinse it, and then return it. Routine stirring will prevent the need for drastic cleaning and maintain the attractive appearance of the display aquarium.