Using rocks in the waterscape
Using rocks in aquascaping. Rocks are an integral element of your aquascape. They lend visual stability and permanence, they define fish territories, and provide cover. Rocks help you effect changes of level. They may mask the line where the substrate meets the rear glass, which should never show.
Takashi Amano blew great gusts of fresh inspiration into European and American planted tanks in the 1990s: planted tanks became "aquascapes". Amano's "Nature Aquarium" style draws on classic Japanese gardening principles, in which carefully selected rocks, minutely distinguished and sensitively grouped, represent visions of rugged cliff faces and distant mountains, rising from low canopies of green forest. These effects, as often as not, recreate terrestrial landscapes of distant views in the open air, rather than underwater ones, however. Japanese principles of rock placement may or may not control other scenic effects in creating a sense of place in the aquarium, for not all rocks in aquaria need strive towards iwagumi, the Buddhist stone triads that were the basis of stone and gravel Zen gardens, first codified in the 11th-century gardening text Sakuteiki.
Cliff faces. 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. See the rocks illustrated above and 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. Of course, as a result there's a plastic milk crate full of rocks sitting under my bed...
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 cement forty-eight hours to cure. With silicone there should be no lingering vinegary acetic acid smell. With the rocks placed, sitting stable on the tank's glass bottom, 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 or a plastic cup with many holes punched in the base with a hot nail to stand directly on the bottom glass, securely taped into an accessible front corner. Alternately misting all the rock and gravel surfaces and siphoning from below will draw the fine sediment down into the substrate where you want it.
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. You'll always accumulate enough of what masons call and chinks, like the small ones in the photo above, which help fill in gaps or stabilize larger rocks.
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. Or, 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.)
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! You detect that I'm traumatized still by an old catastrophe of this kind. 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; they give you some "plenum"-type de-nitrification. Build up caves by laying slate sections on one another, like open brickwork. A dot of the two-part epoxy aquarium cement that hardens underwater will help hold the structure together. Silicone aquarium cement will swell up with water and may become visually obtrusive if you use too much.
Don't depend on cement for stability: a stable rock always sits steady on three points. Chips of slate can be slid in from behind under a rock until it sits tight.
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 easier to grip. 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. But the water ferns Java Fern and Bolbitis require that you keep their thin rhizomes from getting buried; they need to be lashed to rocks or wood.