Iron sand (aka black sand) is mainly composed of magnetite is a dark black sand occurring on many beaches around the world (I don't know if it occurs in any freshwater systems but there may be overlap)(is very similar if not
@seangee 's sand).
I agree with your statement here, yet a couple of posts back you said
you then reply to my message in less general terms specifically about corydoras yet this is a very broad statement. Without going into detail about corydoras or similar bottom dwellers what is your basis for this? Define what you mean by black sand because I have a feeling our ideas of it are quite different.