Alcohol does not work at a preventative spray for spider mites, it will kill them on contact but there is no residual effect against pest mites. You usually have to do 2 - 3 back to back treatments to kill the newly hatched mites.
For fungus gnats, the sand does not act as glass. It keeps the adult females from laying new eggs in the soil of the plant. This does not stop the damage that the larva is doing to the roots of your plants. Fungus gnat larva eat the roots of your plants, stressing them. The adults can vector plant diseases so it is very important to control them. They adults can be trapped using yellow sticky cards. To control the larva I use ( and professionals) use beneficial nematodes.
As for as using hot water, fungus gnat larva get into the roots of plants. If you have water hot enough to kill the larva in the roots / you are going to cook the roots of your plant. Again beneficial nematodes offer an excellent control method for fungus gnat control. Once applied it takes 1-2 weeks to get control and they are organic.
I wrote a few articles on fungus gnats and you can find them at bugladyconsulting.com/Publications.htm
