After the aquarium is in place the first consideration is the sand. Cleanliness is of utmost importance. New sand should be washed thoroughly until all foreign matter and impurities are removed. Beach sand, which comes in various textures and colors, can be used. There is always some contamination in beach sand such as bits of dead crustaceans or seaweeds which must be removed. Thorough washing not only does away with much of the foreign matter but it also removes the dust-fine particles which would otherwise mar the perfect clarity of the water.
A good way to wash sand entails the use of container (plastic bucket and an ordinary garden hose. Place the container where the overflow won't hurt anything and regulate the stream of water from the hose so that you can stir the sand up vigorously without splashing too much or washing the sand itself out of the bucket. For best results keep the bucket only one-half to one-third full of sand. Poke the nozzle of the hose into the sand and move it around until nothing but clear water flows out over the edge of the bucket. Drain the sand thoroughly. If time and space permit the sand should be spread out on a flat, clean surface to dry in the sun. The sun's ultra-violet rays will help sterilize the sand to some extent.
The coarseness of the sand is important. In fresh-water aquaria the substrate is used to support rooted plants and must be chosen accordingly. In marine aquaria the particle size should be selected with two things in mind. Firstly, if a sub-sand or undergravel filter is used the sand should not be so fine as to fall through or clog up the slots. If fine sand is desired a coarse layer can be placed between it and the filter. Secondly, coarse sand might trap bits of uneaten food and detritus that fall onto it and remain there to decay and cause foulness. If a sub-sand filter is not present fine grain sand that packs tightly can be used. All foreign matter should stay on top from where it can easily be removed.
The depth of sand has to be considered. A deep layer is more likely to become contaminated than a shallow one. All that is needed in an aquarium is about one-half inch or so, that is, enough to cover the bottom unless you have burrowing fishes in your tank. On the other hand a sub-sand filter requires a deeper layer of sand. The types of fishes kept have to be taken into consideration as well. Most marine fishes do not need any great depth of sand and the half inch will suffice. even flat-fishes lie on the surface with only a light covering over the edges of their fins to provide protection through camouflage. Wrasses and eels are great burrowers and dive into thw sand when danger threatens. For these fishes a greater depth of sand must be provided depending on their size.