here is nothing wrong with using silica sand, however, it provides absolutely no buffering capacity. It is also very fine and although that could be a good thing for silt and such, it is also very jagged and sharp which is not good for most burrowing inhabitants. I would say the main disadvantage is buffering capacity. If it holds none, it just means you will have to supplement more to acheive more. There have been many a tank successfully reared with the use of silica sand alone and suffered no ill effects. Check it out:
The silicon in the sea, like phosphorus. originates from the land. Silicon (Si) is the element. When compbined with oxygen, it becomes silica (silicon dioxide, SiO2), the stuff of sand, flint, quartz, glass, and the skeletons of some tiny planktonic protoctists. Silicone ( with the "e") is SiO that is combined with various semi-inorganic polymers (R groups) to form adhesives, lubricants, and coatings.
Some silica is dissolved during the breakdown of rocks and sediments and is washed into rivers and out to sea. Silicon is an esential element to animals that use it in the development of skeletal supports such as diatoms, radiolarians, silicoflagellates, and sponges. In the marine environment, silica is obtained from the water by the organisms that utilize it, and some is returned to solution when the organism dies. Silica that is not dissolved, accumulates as sediment on the ocean floor.
Silica in the form of sand, rock, and glass is not very soluble and is not a chemical concern to marine aquarists. Silica in the form of silicic acid is another matter. Very few animals are that require dissolved silica in any significant amount; however, excess dissolved silicic acid may stimulate growth of unwanted diatoms and certain red algae. Silicic acid does not usually accumulate to problem status in marine systems. In fact, if anything, it is depleted from aquarium water. However, if fresh water supplies contain high levels of silicic acid, constant addition of tap water to make up for evaporative loss may cause excessive diatom growth is other nutrients are also available...
From the words of MOE