Why does sand in your aquarium turn black?

The black is Bacteria build up. This always happens in nature and should be OK provided you keep to the basic rules that follow and also have plants growing in the sand. The basic rules for successfull fishkeeping are as follows. :- 1 inch (2.25cm) of fish needs at least 1 gallon (3.78ltrs) of water. Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week. If you follow the above rules, feed the fish the correct diet and keep the container covered and at 75F most tropical fish will do OK. I can guarantee that if you don't keep the preceeding rules that your fish will have constant health problems and will live a considerably shorter life.