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.
That strongly implies a type of mold or algae. Empty the tank and clean everything in it - and the tank itself - with a solution of bleach and water. Rinse everything multiple times, then put it back into service.
Its the rock, sand, gravel, whatever on the bottom of an aquarium.
You mean substrate?
because the fish want it
No you cannot. It will turn the sand and water black.
I have a 120 Gallon aquarium with sand. They love it.
a terrarium empty.
sedimentation
Yes ofcourse you can have them in your saltwater aquarium. You have to provide some space in your tank with good sand and feeding sand dollars is easy, for they mostly eat suspended organic matters and other fish wastes.
Sand Masters - 2011 Aquarium of the Pacific 1-12 was released on: USA: 21 August 2011
The black plastic around the edges of the aquarium are simply referred to as 'trim'.
yes you can
Yes sand can turn into glass