Aquascaping and Substrate

The aquascape is how the rock and sand are arranged in the display. It has four jobs. It must be stable, so nothing shifts or falls onto animals or glass. It must let water flow everywhere, including behind and under the rock, so waste stays suspended for the filters (Module 13). It must give the animals the right place to live: the right surface, light and space to grow. And it must be easy to maintain, so every spot and all the glass can be reached.

The rock and sand are also part of the biological filter (Module 8). Build with the end goal in mind, not just how it looks on day one: animals grow, and the scape has to leave them room. Live rock brings its own life into the system and seeds the sand with it (section 2).

Live rock is rock taken from the ocean, full of bacteria and other life, which brings biodiversity into the system. It can also carry pests, unwanted algae and disease, which Modules 23 and 31 cover. Cured rock has been kept in water away from the display until it stops leaching and grows bacteria (Module 9). Dry rock is clean and dead, mined or man-made: no pests, but far less life, and it can gain some life over time without ever matching live rock.

Choosing live rock means accepting both what it brings and anything unwanted that comes with it. In return, the sand and the whole system fill with a wide variety of beneficial benthic organisms (animals and microbes living on and in the bottom), which spread from the rock on their own. Dry rock gives far less of that, which is a strong reason to build reef systems on live rock (Module 36).

Live rock is built by stacking it properly so the pieces interlock: each rock sits in a stable position and is locked in place by the rocks around it. Live rock can't be glued, and mocking a scape up outside the tank only works for dry rock. Start with a solid foundation: the bottom rocks go on the glass, on egg crate (a plastic grid) laid on the glass, or on weighted bags of sand or crushed coral hidden by the substrate. Sand moves, so a base set on top of loose sand can shift.

Dry rock can be laid out on a table at the tank's own size first, glued, epoxied, mortared or drilled and threaded onto acrylic rods. Even then, glue and epoxy aren't structural on their own; the fit comes first.

Whatever the rock, keep heavy rock low and stable, and lean nothing against the glass. Loose stacking is a common cause of collapse that endangers the animals and the glass, and in an earthquake tall, loose rock can fall against the glass (Module 10).

Leave open space behind and around the rockwork, and keep it off the back glass. Rock pushed against the glass, or a solid wall of rock, makes dead spots where flow stops and detritus settles and rots. Standing waves and pumps aimed across and under the rock reach those spaces (Module 13).

Leave room to work. Rock too close to the walls stops anyone scraping the glass, and a scape packed tight can't be maintained. Before a scape is finished, check that every pane of glass, every pump and every animal can be reached.

Giant clams differ by species. T. crocea bores into hard surfaces and is never found on sand; T. maxima lives on hard surfaces too. Hippopus clams live on sand or gravel. T. gigas attaches to rock. T. squamosa and T. derasa attach to hard surfaces when small and often let go later, so either works. Place a clam on a surface much bigger than the clam, with room to open fully and to grow. On rock, a clam can drop into a gap or onto a stinging animal, and steep or narrow spots make that more likely. Clams attach with byssal threads; never pull an attached clam, because pulling can tear out the organ that makes them. Cut the threads close to the surface instead (Module 28).

Carpet anemones live in sand. S. haddoni needs a deep sand bed, at least 10 cm (4 inches) and better 15 cm (6 inches), and can pull itself fully under the sand. S. gigantea lives on shallow sand flats and needs a sand bed too. Anemones move, so cover overflow and pump intakes with a sponge so an anemone can't be pulled in (Module 29).

Corals follow general rules. SPS (small-polyp stony) corals usually want stronger light and stronger flow, so they go higher in the scape, low enough that they won't reach the surface as they grow. LPS (large-polyp stony) corals usually do better with moderate light and moderate flow, so they go in the middle or lower, some on the sand bed. Many LPS have long sweeper tentacles that can reach 15 cm (6 inches) or more and kill what they touch, so give them plenty of room from other corals. Where two currents meet, the flow is stronger and can knock a coral loose. Leave every coral room to grow (Modules 13, 18 and 27).

Reef sand is usually aragonite, a form of calcium carbonate, and comes in grain sizes from fine to coarse, up to crushed coral and gravel. Fine sand has a lot of surface area for bacteria, but in strong flow it blows around and leaves bare patches. Coarse sand is heavier and stays put.

A wide variety of substrate sizes and depths can be used, depending on the goal, the species kept and the biotope (the natural habitat being recreated). Carpet anemones and sand-dwelling clams need sand, and so do animals that sift or burrow in it. Live sand is sold or moved with bacteria already on it, for a faster start than dry sand (Module 8). With live rock, the substrate also fills with beneficial benthic organisms from the rock on its own.

A shallow sand bed is about 4 to 5 cm (1.5 to 2 inches) of fine sand. Its bacteria mostly live with oxygen and handle ammonia and nitrite. With fine grains, oolitic (very fine, rounded) up to about 1 mm, a bed counts as a deep sand bed from about 10 cm (4 inches); many deep beds are 15 to 20 cm (6 to 8 inches). Bacteria that need oxygen work in the top 5 to 8 cm (2 to 3 inches); below that, bacteria that live without oxygen turn nitrate into nitrogen gas, which leaves the water (Module 8).

A deep bed takes a lot of sand and space. Phosphate can build up in it while nitrate falls, shifting the balance between them (Module 6); that matters most where calcifying animals live. Stagnant areas in a deep bed can form pockets of poisonous hydrogen sulfide, which is why a deep bed needs animals that keep the sand moving (section 8). Shallow or deep, fine or coarse, the right choice depends on the system's goal and its animals.

Reef systems never have their sand siphoned or vacuumed. The fauna keep the sand clean: animals that stir, sift and eat in it, such as nassarius snails, conchs, sea cucumbers, sand-sifting gobies and some starfish, plus the benthic life that spreads from live rock. Siphoning sand is only for systems meant to be more sterile.

Match the cleanup crew to the substrate: its grain size, its depth and the animals already there (Module 24 covers choosing them). A deep bed especially needs animals that keep it moving, because stagnant deep sand full of detritus can cause serious problems. Waste building up on the sand means the cleanup crew is missing, too small or wrong for that sand, or the flow isn't reaching it (Module 13). Bare patches in fine sand mean a pump is blowing it around: aim the pump across or above the rock instead.

Rock moved or fell: check how it was stacked, the foundation, and whether there was an earthquake; restack so the rocks interlock, heavy rock low. Detritus collecting behind rock: a dead spot; open up the rock or add flow there. Sand blown bare: a pump aimed at the sand. A clam that fell into a gap: its spot was too steep or narrow; give it a flat surface bigger than the clam. An anemone at an intake: turn the pump off, ease it free and cover the intake. Waste building up on the sand: the cleanup crew is missing or doesn't match the sand.

Look at the aquascape in the daily check: anything that moved since yesterday is a warning. Change one thing at a time and watch the result (Module 1).