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Re: rocks in the aquarium

Robert H wrote:

> I think you partially answered my question on oxides, but can
> you anser this: how much of a concern is rocks leeching their metals into
> the water?

For most rocks there is no concern at all.  The metals (primarily
aluminum and iron) are bound mostly in stable forms that won't dissolve
or break down in an aquarium.  The exceptions are mostly the "ore"
minerals like pyrite, galena and sphalerite.  These don't dissolve
directly but can break down slowly.  Unfortunately these are also often
attractive rocks, so there's a temptation to stick them in an aquarium.

> Can plants utilize iron oxide from rocks, without it being broken
> up into soil where it reacts to organics, and are minerals such as zinc or
> copper that may be deep within the rock pose any threat?

Generally no to both questions as long as you avoid the metallic-looking
sulfide ore minerals.  Plants need iron in only minute concentrations,
so any amount in the water is useful to plants, but most of the iron in
rocks is in such stable forms that it won't get into the water in even
minute quantities.  Copper and zinc (just to name a couple) are rarely
present in rocks at signficant levels; avoid using odd-ball rocks or
ores and you will never need to worry about them.

> And lastly, what
> other minerals besides calcite can affect pH? I think you mentioned one
> already.

In addition to calcite, dolomite and any other carbonate mineral can
raise alkalinity and pH.  There are many carbonate minerals, but only
calcite and dolomite are common.  In fact, calcite is nearly ubiquitous,
so it can be difficult to avoid.  Usually it is enough to avoid rocks
that don't contain calcite as a major component.

Sulfide minerals don't dissolve directly but they can react to produce
sulfuric acid, which drops the KH and pH.  The reaction is typically
fairly slow.  Pyrite, in particular, can be present as a part of many
kinds of rocks.  Small amounts generally won't cause a problem in a tank
with well-buffered water.  Just the same I prefer using well-weathered
stones from which the pyrite has already been removed.

There are other odd-ball rocks and minerals that may effect pH.  Some of
those can be found in a rock shop but otherwise you won't see them.  If
you do want to buy rocks in a rock shop, then you should tell the clerk
what you want to do with the rock.  He or she may be able to steer you
away from problems.

Roger Miller