It depends how big your aquarium is. I would start with a few bags cause you can always buy more, Just work your way up until you think you have enough sand.
Yes, sand does very well in a goldfish tank. In fact, it helps goldfish break down food when sucked in with it. It also makes clean up much easier if you use a siphon or baster.
You can safely put 2ml into the 10l aquarium
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.
Its the rock, sand, gravel, whatever on the bottom of an aquarium.
You mean substrate?
because the fish want it
I have a 120 Gallon aquarium with sand. They love it.
a terrarium empty.
sedimentation
Sand Masters - 2011 Aquarium of the Pacific 1-12 was released on: USA: 21 August 2011
yes you can
Your filter will tell you on the side of it how much media sand to put in it.
a lot because they get the water from the sand so just go to a mudflat thing and get a handful of waterlogged sand