Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have some stone in a tank that has rust spots. They do not come off like that. A tiny bit might brush off, making a momentary cloud, but the main spot stays there and keeps on rusting.
The fish are just fine.
Now, this is not a lot of rust- a few nickel and dime size spots in 125 gallon tank.
I have seen some forms of slate that do have a LOT of rust (I do landscape design and installation). A rock that is that bad will be seen to be that bad in the store: It will already have some suspicious areas and when they get wet the rust is all over the place. Whether this is actually, geologically, slate I have no idea. It is rock that is cut into thin pieces and mortared onto walls, patios and so on. Then it rusts, staining everything under it.
I am with the other people and yourself: More likely diatoms or something.