Substrate — What You Put on the Bottom

For a general introduction to the physical environment, see The Environment; for the habitat that any bottom creates, see Surfaces. This page is about the single most consequential setup choice after water and light: what covers the floor of the tank.

The simulator gives you four bottoms, and they are genuinely different ecosystems — not cosmetic skins on the same model. From left to right they trade cleanliness for life:

"Inert" Means Inert

The two middle options are chemically inert: pure silica sand or quartz gravel with zero organic matter and zero carbonate. This is an important distinction. A bed of crushed coral or aragonite is also organic-matter-free, but it slowly dissolves and pushes up your GH and KH — it is a buffering substrate, not an inert one. Inert sand and gravel do nothing to your water chemistry. They are scaffolding: a place to root, a place to graze, a place for mulm to settle. Any fertility in an inert tank comes from the water column (your dosing, your livestock waste, your fish food), never from the grains.

That single fact — inert beds feed nothing — is what separates them from active soil, and it ripples through everything below.

The Floor as Habitat

The moment you add any substrate, the bare-glass jar becomes a place to live. A floor surface is colonizable real estate: periphyton (the brown and green films), biofilm bacteria, nitrifiers, and the small benthic grazers that crop them. See Surfaces for how a surface's texture and carrying capacity set how much life it can hold.

Sand and gravel are different habitats, and the difference is real in the model. Gravel is the richer, more stable bottom: its coarse, locked-together grains hold a denser, longer-lasting biofilm than shifting sand, so a gravel tank tends to carry more periphyton — and more of the grazers and nitrifiers that periphyton feeds — than an otherwise identical sand tank. Sand, with its finer texture, suits the smallest interstitial grazers (the ostracods and copepods that pick between grains) better than gravel's larger gaps do. Neither is "better"; they favor different residents.

Sand Breathes, Gravel Doesn't

Here is the quietest, most important difference between sand and gravel, and it is pure physics.

As a tank ages, dead leaves, uneaten food, and waste settle into a layer of mulm on the floor. Bacteria decomposing that mulm consume oxygen. How deep oxygen can reach into the pile before it runs out depends on how easily it diffuses through the grains — and that is where grain size matters. Oxygen threads through coarse gravel easily, so even a thick mulm layer stays oxygenated to the bottom. In fine sand, the tortuous, tightly-packed pores choke off oxygen, so the underside of an accumulating mulm layer goes anoxic.

That anoxic micro-zone is where denitrifying bacteria do their work: starved of oxygen but surrounded by nitrate, they breathe nitrate instead and exhale inert nitrogen gas, which bubbles away. So an aged sand bed quietly removes a little nitrogen from the system — a gentle, natural counterweight to the nitrate that nitrification keeps producing — while a gravel bed, staying oxic, removes much less. The effect is modest (the grains differ only about 25 % in how fast they pass oxygen), but it is real, it is grain-aware, and it grows with the mulm. This is the same physics that makes a deep soil bed denitrify; an inert bed just needs accumulated mulm rather than buried dirt to get there.

What inert beds don't do (yet): because an inert bed carries no buried organic matter of its own, it never drives the deeper, smellier end of sediment chemistry — the hydrogen sulfide and methane of "old tank syndrome." Those need the relentless oxygen demand of a real organic soil layer. In the current model an inert bed denitrifies but never turns sulfidic, which is a fair picture of a clean, well-kept inert tank.

Can You Root Plants in It?

Yes — but how well depends on the grain, and nutrition and anchorage are two separate questions.

Nutrition. Rooted plants in active soil feed their roots directly from the nutrient-rich pore water — that is the whole point of a dirted tank. On an inert bed there is no pore nutrition, so a rooted plant falls back to feeding entirely through its leaves from the water column. That works (this is exactly how high-tech inert-substrate tanks run, on water-column dosing or root tabs), but it means a rooted plant on inert sand is only as fed as your water column is.

Anchorage. Separately, the grain decides how well a plant can physically grip and establish. Fine sand anchors a crown as firmly as a soil tank's sand cap; coarse gravel lets crowns work loose, so plants establish and grow more slowly (the hobbyist truth that most stem and rosette plants sulk in bare gravel without root tabs); and a bare glass bottom gives nothing to root into at all, so a rooted plant planted on bare glass simply cannot establish and declines. The simulator models this as an anchorage factor that scales a rooted plant's growth — full on soil and sand, reduced on gravel, zero on bare — on top of the water-column feeding limit.

So the rooting ladder, best to worst, is: soil (anchored and fed) → sand (anchored, water-fed) → gravel (loosely anchored, water-fed) → bare (can't anchor). You can still choose a rooted plant on gravel or even bare in the setup wizard; it will just warn you and model the penalty rather than forbid it.

A Place to Hide

One more thing a real substrate gives the tank: a place for the slow-growing nitrifiers to hide. Snails, shrimp, and other grazers crop biofilm off exposed surfaces, and that grazing can, in a heavily-stocked tank, crop the nitrifying community hard enough to stall the nitrogen cycle. But a grain bed buries much of the nitrifier biofilm down in the interstitial spaces below the grazed surface skin, where radulae and mouthparts can't reach. Fine sand packs tightly and shelters the most; coarse gravel, with its larger gaps, shelters somewhat less; a bare floor shelters none. This burial refuge is part of why a tank with substrate cycles more robustly than a bare-bottom one under a grazing load.

Choosing

• Bare — for a quarantine or hospital tank, a shrimp-only setup you want to keep immaculate, or any tank where you'll float or attach every plant. Simplest to vacuum; no rooting, no benthic life.
• Inert sand — a natural, planted look on water-column dosing; anchors plants well, hosts a fine-grained benthic community, and develops a gentle denitrifying capacity as it matures.
• Inert gravel — a classic community-tank bottom; the richest inert benthic habitat for invertebrates, stays oxygenated, but lean on root tabs if you want heavy root-feeders.
• Active soil — for a true low-tech / Walstad planted tank: the substrate is the fertilizer, roots feed from below, and the full sediment redox chemistry comes alive. Pick a soil type to set how fresh and how rich it is.