Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Buy rocks at a landscape, masonry or rock yard. MUCH cheaper. You can usually take home a free sample and run a few tests, like keeping them under water for a few weeks.
Iron in rocks that shows up as rust is not in a form that plants can use. Perhaps some microorganisms can use it, alter it, then plants can use it.
Iron fertilizer that you add to the tank does not do that to rocks. The rocks have a vein of iron in them. In small amounts it is not a problem, but aesthetically it can be pretty bad. If I was looking for black rocks like that and had so much rust show up I would return them.
I have a few rocks in one tank that have a little rust on them, but it is very little. A few spots under an inch (about 1-2 cm) diameter in a 125 gallon tank.