Sandstone can also be available at your LFS or the stone yard. Sandstone is formed of quartz and feldspar grains mostly, cemented under pressure in a muddy matrix. Sandstones vary as widely in color as shales and slate. Rock buyers for your LFS seem to go for the most bizarre striped sandstones.

The LFS may also carry some petrified wood. Good chunks of petrified wood retain a lot of the wood's original grainy character. Generally, petrified wood grain is as strongly directional as a bundle of arrows. Since the grain is pointing in a direction, you are more likely to want to position it to point up and inwards, towards the central area, so as to concentrate your composition.

There may be some quartz or quartzite, recognizable by their crystalline, somewhat glassy look. They come by their glassy look quite naturally, since quartz is largely silicate, and silica melts into glass. Quartzite is naturally reconstituted silica sand, refused under metamorphic heat and pressure. I find quartz difficult to incorporate. A piece of quartz in an aquarium setting always draws attention to itself. It usually seems pale and foreign.

At the LFS, you may even see some fancy agatesand other precious-looking stones. I always avoid those myself, because I like to try for a "slice" of water representing some place in the tropical world, and not the "Captain Nemo's Parlor" look of marbles and agates and "rainbow rock." Judge the rocks with a critical eye: if phrases like "eye-catching," "collector's item" or "paperweight" come to mind, it's probably the wrong rock.

Takashi Amano says: "Don't line up rocks of the same shape or size; don't use differently colored rocks or rocks from different regions--— and don't use a rock in a way that contradicts its essence." Whew! easier said than done, Takashi-san!

"Aquarium-Safe" Rocks. What's the right rock for your aquarium? Everyone you ask seems to be undermining your confidence about what kind of rock is "aquarium safe." People will warn you to boil rocks you may have found. That's a harmless adventure, if you must. Make sure the internal temperature reaches 180^oF heh heh heh. But don't let them scare you into bleaching your rocks.

Remember that basically all rocks are "aquarium safe" with a few exceptions: calcium and magnesium-bearing rocks, for example. Limestone (which is basically calcium carbonate) and dolomite (which adds some magnesium carbonate) often have a soft and chalky look. They are easily scratched with a knife (less than 4 on the Mohrs hardness scale). Sometimes your thumbnail will even leave a mark. Often these sedimentary rocks reveal their striated bedding planes. But sandstone can have some "lime" content too. Imbedded fossil shells and shell fragments are a giveaway, if you find them. Remember that the minute shelly debris that forms limestones may have been mixed with various amounts of muddy silt, bearing organic materials that make the limestone dark, or with yellow and brown iron oxides, so the resulting lime-bearing rocks may not always be as blinding as the White Cliffs of Dover. The calcium and magnesium content of these rocks will slowly dissolve in water, the dissolving carbonates increasing the buffering and raising the pH. So calcium- and magnesium-bearing rocks are undesirable--— except in your Rift Lake aquarium, where you want that alkalinity. The "honeycomb limestone" you can find in central Texas gives a vivid impression of the way acidic groundwater can dissolve away carbonate rocks. Marc Elieson illustrates these bone-white limestones like calcified sponges in his Rift Lake aquaria, where they contribute to the carbonate buffering. (Don't neglect the other articles at Marc's site.)

What about marble? There will also be lots of marble and marble chips at the landscaping outlet. Marbles are "metamorphosed" limestones, "remade" so to speak, deep in the earth under heat and pressure. Marble will also raise the pH, but it reacts with the acids in water more slowly than limestone. As a general rule, the minerals in metamorphic rocks are more tightly bonded and aren't going to leach into the water. Marble can be any color; my problem with white marble, I guess, is its whiteness. At first it looks artificial and then it looks dirty, tempting you to scrub it.

The tufa rocks at your LFS are often used in Rift Lake set-ups. "Tufa" is loosely and confusingly applied. Tufa may be chemical sedimentary rock composed of calcium carbonate, which has been formed as an incrustation around lime-rich springs. Or "tufa" ("tuff") may be a generic term for pyroclastic volcanic ash solidified into rock. This is sometimes locally called "lace rock." Calcareous tufa will surely raise the pH. Volcanic "tuff" may not be inert either, especially in soft, acidic waters.

Porous travertine has also originally been precipitated in lime springs, as tufa. Though it has been partially metamorphosed by heat and pressure, it still bears a high calcium content that renders it unsuitable in general-purpose aquaria.

Quite apart from tufa, but sometimes confused with it because both are porous and because some tufa is volcanic in origin, is "lava rock," actually a reddish volcanic pumice, the solidified froth of lava. Pumice is inert; it won't affect the pH. Pumice is full of intricate channels; in fact some naturally-occuring pumice is even buoyant and useless in the aquarium. Lava rock is also available, and cheaper, at your local Yard Yard.

Pumice is aquarium-safe. In fact you may get some side benefits from using it: after it has been in place for six months or so, you may witness some slow decline in nitrate in the aquarium. This is due to de-nitrification carried on by certain bacteria, deep in recesses of lava rock, where the water is anoxic. If you pull out the rock and drain it, or worse, if you dry it, the de-nitrifying effect will be cancelled. So, treat matured lava rock as respectfully as you would a matured sponge filter. Its rather harsh and blank brick-red color is improved in time with some algal growths.

If you think your fishes will scrape themselves on the rough surface of lava rock, you can cover it with Java fern, which will root into it without compromising its slowly developing de-nitrating qualities.

Igneous rocks are also chemically very stable. Obsidian is a shiny black volcanic glass, too sharp-edged for the aquarium. Also too shiny and too black. Granite is always good. Gneiss is nice.

Ore-bearing rocks? Ordinarily it would take some pretty strong sulphuric acid to release the ore from the rock, but stay on the safe side. Don't use iron pyrites, or that glittering rock with the sulphur crystals in it you picked up at the company picnic at Soda Lake Springs! Avoid very soft rocks: if the iron oxide is falling out of the rock, or if you can almost scrape it with your thumbnail, let it be.


How to test for carbonates. Everyone you ask will recommend the old trick of putting a teaspoon of vinegar on a suspect rock; if it fizzes it's going to raise the pH. That would be clear enough, but what if it doesn't produce a bubble?

Some prep is necessary. PaulK posted at AquaLink, 13 Aug 2003:

"The acid test would depend on the strength of the acid, but would also depend on the surface you are testing as well. In order to be accurate, a fresh surface should be tested. It is possible that the rock may have a carbonate residue, which would effervesce if acid is applied, though the actual rock itself may be something that contains no carbonates.

On the other hand, a rock may be a carbonate and yet may not effervesce readily when acid is applied. Dolomite, for example, must be powdered to react with an acid."

Some aquarists test using muriatic acid, which is a 20% solution of hydrochloric acid, available at your hardware. For heaven's sake be careful with this intensely caustic stuff.

Here's a better idea: if you have a nitrate test kit, you may also already have a better test than vinegar for lime content in rocks. My Aquarium Pharmaceuticals Nitrate Test contains two bottles of prepared solutions. Solution #1, labelled "Caution: contains hydrochloric acid," comes in a handy squeeze dropper bottle. If a drop or two on a candidate rock fizzes, or even bubbles, that rock would raise the pH in the aquarium.

Fake rock and molded backgrounds. Maybe you'd expect me to be snide about fake rocks. After all, there are those plastic rocks that snap together like a Lego kit to form gravity-defying Rococo fantasy structures rivaling the ormolu rockwork of a French mantel clock, or the setting for a Road Runner cartoon. But frankly I've seen some astounding and great fake rocks cast in polyurethane resin and sponged with color, which are amazingly realistic. That's the material they're using in public aquaria. You can drill this inert material for plumbing or cut it to suit your needs. Filter inlets and outlets can be disguised. Heaters can be hidden too, as long as you've made plenty of allowance for sufficient water current to keep the confined area from overheating.

Have a look at these fake rockwork backgrounds, cast in a polyurethane resin molded on real rock and roots! The Swedish guys who invented this stuff call them "Back to Nature" Aquarium Backgrounds. Several kinds of full backgrounds, or modular rock sections you can piece together, which were briefly available in the U.S., are being sold here again, through www.herpsupplies.com in Campton, NH. For good pix of the various rock walls and Amazon root backdrops molded in polyurethane resin, though, you'll want to look at the Back-to-Nature website. Walter Soestbergen installed one full rockwork background in his "Discus Page Holland"--— Eat your heart out!

Joakim Andersson offers several thoroughly illustrated step-by-step guides to building your own backgrounds, using concrete, plastic resins, styrofoam, etc., plus photos and tips on using the inspiring Swedish-made "Back-to-Nature" modules. It's all at "Cichlids Inspiration,"

An excellent step-by-step, fully illustrated description of a styrofoam "rock" background by Jay Luto was posted at www.aquabotanic.com 19 May 2003. Jay used pink styrofoam rather than white, with additional chunks siliconed to it, and West System two-part epoxy mixed with dye, in multiple coats, sprinkled with sand between coats.

Caves. Rockwork caves have to be stable. You know that. But it's amazing what a determined female Apistogramma cacatuoides can accomplish in undermining rocks that rest on gravel, by moving one grain of gravel at a time! So, even if your earth-moving dwarf cichlids will max out at an inch and a half like mine, always lay out the rockwork before you put any gravel in the bare tank. Start with the thick flat rocks that will lift your visible cavework above the finished substrate level. Or base your rockwork on lighter and cheaper plastic "eggcrate" light-diffusing panels that will equalize the weight on the tank's bottom glass. Cell-Pore slabs will also serve in this way, while they give you some "plenum"-type de-nitrification. Build up caves by laying slate sections on one another, like open brickwork.

A dot of aquarium cement will help hold the structure together. Don't depend on cement for stability: a stable rock always sits steady on three points. Remember silicone will swell up with water and may become visually obtrusive if you use too much.

Rocks that have one corner with an acute angle (sharper than a right angle) can be set so as to present a vertical front surface, combined with a top face that slopes back into the gravel bank behind. Get my drift by looking at your own tight-clenched knuckles, or think of the figure 7 tipped onto its side. This combination of faces keeps the gravel retained behind the rock face from slowly eroding out of your high terrace. I often look over the supply of LFS rocks in my favorite red-brown color (the color that matches my gravel mix), ever on the lookout for these useful acute angles, even when I'm not actively setting up an aquarium.

When setting rocks, keep your bedding planes close to horizontal. Bedding planes are the visible strata of sediment that were laid down one after the other to form the rock. Bedding planes were perfectly horizontal when they were deposited and remain horizontal, unless geological stresses have tipped them into a geologist's "syncline." Horizontal bedding always looks more inert and more stable. If you are tilting rocks, keep the angle of your slant consistent all through the tank. Don't upend flat rocks unless temporarily, when you're spawning Angels; rocks tipped on end invariably look like artificial "rockwork" --and even worse when a flat rock is laid across them to form a Flintstones' "cave."

While you're setting rocks, from time to time replace the lighting reflectors as you work, so that you can adjust the light-and-shade effect the rock is going to have.

When it's all to your liking and structurally sound, give any dabs of silicon cement forty-eight hours to cure. There should be no vinegary acetic acid smell. Then add the lower level of your substrate mix. Flush it into place by gently spilling water on it, to fill the spaces among the rocks. Then siphon out all the water, which will be very turbid with rock flour, before you carefully re-fill the tank. You can do this siphoning most thoroughly if you have inserted a short section of tubing to stand directly on the bottom glass, at an accessible front corner. Siphoning will draw the fine sediment down into the substrate where you want it.

Terracotta pots. Pots make fine caves. Cutting terracotta pots for aquarium use without smashing them to useless shards with a hammer takes some finesse, though. You can secure a pot in a table vise and saw it apart with the kind of small-toothed blade you'd ordinarily use for metal. Get a good grip with needle-nose pliers, and you can snap off sections. Don Zilliox aka "Z-Man" uses a round carbide sawblade in a regular Stanley hacksaw frame.

It's not good practice to reuse flowerpots that have done service in the garden, because they may contain fertilizer or pesticide residues that could leach into aquarium water. To soften the raw color of a fresh pot, though, try growing Java Moss over it, using the technique I use for a moss lawn covering coconut shells.

Flowerpots soon develop a softening coat of algae.

Selecting water-worn stream cobbles. One thing you'll rarely see among your local fish store's collection of rocks: a rock that shows any sign whatsoever of having been worn by water! The landscaping outlet will be a better source for worn cobbles. A few large water-worn pyramidal cobbles can break up the flat base of the aquarium box. A tank "floor" with naturally-varied levels that looks like a streambed is not an easy effect to achieve, as you have learned from all the failed gravel terraces you've attempted over the years, eh? But you might have to seek out water-worn cobbles yourself, in a local stream.

I think people worry unnecessarily about the safety of rocks out of a possibly polluted stream. Rocks won't absorb pollutants. Of course, when you select your stream it won't be right below the paper mill's effluent either, nor downstream from the old uranium mine.

Make your way slowly along a small stream (or through the "landscaping" cobbles at the local Yard Yard) and look for smoothly worn rocks that bear some relation to the shape of a pyramid. Don't waste your time with rocks smaller than your fist, even for a ten-gallon tank. Judge likely rocks on the palm of your hand. The wider the base is, in proportion to its height, the more stable the rock will seem. Stash likely rocks in your backpack. (Not at the Yard Yard, where they'll nab you for cobble-lifting.)

Later, when you come to use a rock with a tapering pyramidal shape in your aquascaping project, you'll completely mask all its lower corners and edges with your mixed substrate, so that it will rise like the tip of a giant boulder that is all but buried in the gravel and loose cobbles of the streambed. As you make your way along the stream, try combining your good pyramid rocks with a second pyramid so that there's a narrow cleft separating them. If a rolling cobble were to come to rest against them, you'd have a cave. If Amazon Sword were planted in the cleft, it would be safe from all but the most determined plant-digger. Keep editing your rocks as you walk. (No, you don't have to put them back exactly where you found them. But in Britain I understand that it's illegal to remove beach cobbles, even from beaches that are made of cobbles. Cobbles all the way down! Too cobbley to lie on.)

Planting rocks. Takashi Amano brought us the idea of wrapping flattish cobbles with Riccia fluitans, which eventually loses some of its buoyancy, or so I hear, and gets a grip on the stone surface ...but never for me, as you might have guessed. Ol' Java Moss may not have the refined texture and bright pale green color of Riccia, but I've had some success lashing Java Moss to cobbles with black cotton thread. By the time the cotton thread has disintegrated, the Java Moss should be attached to the cobble. The rough porous texture of pumice ("lava rock") gives Java Moss something to grip. Java Fern too. Frankly, I've had the best success using smallish shards of coconut shell to make a decorative moss clump that can be clipped over with scissors from time to time.

Snail shells. If your water is hard and gives a pH reading that's already too high, you don't want to compound your problems by decorating with snail shells. But in my very soft water, a large snail shell in a 10-gallon tank helps provide some desirable pH-stabilizing buffer. All mollusk shells are built of calcium carbonate, with a colorfully-patterned outer layer of organic materials--— the periosteum--— that protects the shell from acids. Be aware that if pH lies in the 6.0 to 6.5 range, you'll quickly lose this decorative outermost layer. The shell itself is made in two layers, with an upper layer of crystals of calcium carbonate packed together side-to-side standing on end, so that light is scattered, making this layer appear white, over an inner layer of "mother o' pearl," where the same calcium carbonate is laid in flat overlapping plates that refract light in iridescence. Many small fish will take temporary refuge in an empty snail shell. In my tanks, eroding snail shells change their appearance as they go. Of course you wouldn't want to use obviously marine shells or coral fragments in a freshwater tank. Stick to freshwater snails. The ones sold by a gourmet deli, intended for serving escargots, are just fine.