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Re: rocks for the aquarium

Daphne Freeman wrote:

> I found Robert's info on rocks very interesting but have a question.  I have
> been considering adding some slate to my tank.  I haven't already since you
> couldn't see it though the green water!!  Anyway, it is black slate with
> deposits of rust-red in places. You can rub it off with your finger.  The
> "deposits"  grow in the aquarium and easily crumble into a mess if you rub
> across them in the water.

It's a somewhat-educated guess, but probably your slate contains a
little pyrite in microscopic grains.  Under water the pyrite oxidizes
and produces brownish iron oxide stain.  Small amounts of pyrite in a
rock aren't much of a problem but large amounts of pyrite can be a
problem, especially in poorly buffered water, as the oxidation process
liberates a lot of acid (as in acid mine drainage).

I have a couple other things to add to Roberts' descriptions.

Classically, onyx is a form of silica, and would be safe for an
aquarium.  Most of the onyx I see for sale is actually "Mexican onyx",
which is actually a sort of travertine and composed mostly of calcite. 
It isn't any better for an aquarium than is a chunk of limestone.

Sandstone is made of sand grains cemented together and it's mostly safe
stuff. The cementing material in some sandstone is calcite, which again
isn't particularly good for an aquarium unless you actually want your
hardness and alkalinity drifting upwards.  You might want to give it the
acid test; a sandstone cemented with calcite will fizz under a drop of
strong acid.

Metallic-looking minerals like pyrite (fool's gold) or galena need to be
avoided.  Metallic-looking minerals are usually sulfides, but can also
be arsenides, selenides or antimonides.  They can oxidize under water,
releasing  acids, potentially toxic metals and other solutes.

In fact, mined rocks in general may cause problems because they can
contain small amounts of soluble or reactive minerals that are removed
from rocks -- like stream-rolled stones -- that have been exposed at the

Being a geologist I have a long-standing interest in rocks and I've
often kept exotic-looking rocks in my tanks.  Some are aesthetic
disasters.  Of course, aesthetics is a rather personal matter, so there
are few rules.  There are some pretty simple things to keep in mind,
though.  Mostly, remember that not all colors go with green.

I used to keep a large chunk of jade in one of my tanks.  It never
looked right and after a while it dawned on me that the bluish-green
color of the jade clashed with the chlorophyll green of the plants.  I
also have a tank with a pile of slabs and chunks of bright red jasper. 
Same thing.  It clashes.  But it's also home for an old red-finned
botia, so I leave it there.
Petrified wood is great, and it's generally a safe and interesting
rock.  Incidentally, I think that collecting any petrified wood from
Federal land is illegal and collecting it from the Petrified Forest can
cost you a lot of money.  Some petrified wood is brightly colored, and
some of those shades don't work well with green plants.  I still have
some petrified wood (from Texas) in one of my tanks that is shades of
yellow and orange.  It stands out. The more plentiful petrified wood
from Arizona and New Mexico comes in a fairly wide range of colors, but
many are similarly bright and clashing colors.  If you want to use
petrified wood then it's probably better to use stones with more neutral

I think Amano pointed out in his first book that jagged rocks looked
unnatural in an aquarium, and that rounded river stones produce a more
harmonious effect.  I agree completely with that idea.  River stones
also have the advantage that they usually have fairly smooth surfaces;
there are few crevices and pits for algae to get a hold in where your
algae-cleaning crew can't get them out.  As a result, smooth stones stay
looking better under water then do rough or jagged stones.

There are a few rocks that I've seen for sale in the aquarium trade that
have no business in an aquarium.  Gypsum stands out as a great example. 
It comes in many forms, and at least three forms can be quite beautiful;
these are alabaster, satin spar and selenite.  Alabaster is fine-grained
and usually white or pink. Satin spar is shimmering white or pink
parallel fibers, usually filling a vein in other rocks.  Selenite is a
clear crystal and can be very large.  All of these forms are just gypsum
(calcium sulfate dihydrate), which is quite soluble.  It will gradually
disappear in your tank, and send the hardness sky-high.  It's also very
soft, so all of these forms can be detected because you can scratch them
with your thumb nail.

Rocks.  There's always more to be said about rocks.

Roger Miller