To respond to your last 3 posts:
Substrate is the layer of stuff that is at the bottom of your aquarium, be it gravel, sand, planting medium (special substrates like eco-complete), etc. So sand is one type of substrate.
Master robo answered your question about vacuuming the sand; it takes careful handling to avoid sucking up the sand. Some members have previously posted that they usually do get some sand but don't fuss, they just replace it when it gets low.
As for stirring the sand, this is done not to free up solid waste (which usually doesn't penetrate the sand like it does gravel, but sits on top until it has been broken down by bacteria and then it can penetrate the sand) but more to ensure the sand does not compact. All substrates will compact to some degree, even gravel; this is both good and bad. When it compacts, the broken down waste is acted upon by aenerobic bacteria (simply put, those that do not require oxygen, as opposed to aerobic bacteria which do) that convert it to nitrogen gas that is toxic but usually released at the water surface. There is a process whereby the plants use this, and several related actions are part of it. However, if the substrate, be it sand or gravel or anything else, is allowed to compact too much, "dead' spots form where this anaerobic activity increases to the point of becoming a pollutant and highly toxic to fish. There is varying opinion as to the extent this can kill fish, but one thing is certain and that is that the process occurs, simply because it is part of the natural processes that operate in any aquarium as in nature. Of course, in nature all this activity is usually of little harm to the fish because the ratio of water volume to the fish is far greater than in our aquaria. But in a closed system where the fish cannot escape what occurs in the substrate or the water, the aquarist has to be vigilant to avoid trouble.
Byron.
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