Substrate depth: how deep for plants, fish or nothing at all
Substrate depth looks like an aesthetic choice and is actually a functional one. It sets how much root space your plants have, how much waste collects where you cannot vacuum it, and — in the extreme — whether the bottom of your tank becomes a hydrogen-sulfide factory. The good news: the guidance clusters tightly, and the amount you need is one multiplication away in the substrate calculator.
Key takeaways
- Planted tanks: commonly 5–7.5 cm (2–3 in), often sloped higher at the back.
- Fish-only tanks: under ~5 cm (2 in) is plenty; more depth mainly collects detritus.
- Very deep fine sand (8+ cm) can go anaerobic unless deliberately managed as a deep sand bed.
- Bare bottom is a legitimate choice for quarantine, hospital and some breeding or goldfish setups.
Planted tanks: root room beats decoration
Root-feeding plants — sword plants, crypts, vallisneria — invest heavily downward, and the commonly published guidance for planted tanks is 2–3 inches (5–7.5 cm) of substrate, with many aquascapers sloping from about 2 inches at the front to 3–5 at the rear for depth perception. Hobby substrate calculators (e.g. AquariumCalc) are built around exactly this band. Aquarium Co-Op's planted-substrate guide adds the material caveat: very fine sand compacts hard on roots, while coarser sand and gravels leave root-friendly gaps — depth cannot fix the wrong grain size. Nutrient-rich aquasoils let you run the same depths with less dosing, at the cost of ammonia release in the first weeks (see the cycling guide).
Fish-only tanks: less is more
Without plants there is no reason to build a deep bed. A centimetre or two — enough to look natural and host some bacterial surface — is the practical norm, with ~2 inches (5 cm) the usual ceiling. Every centimetre beyond that is volume you must vacuum, water it displaces (check the volume calculator), and mass on your stand: at roughly 1.5–1.6 kg per litre, a needless extra 2 cm on a 120 × 45 cm floor is about 12 kg of wet weight.
Deep sand beds: real technique, real risk
Marine and some freshwater keepers deliberately run 8–10+ cm deep fine-sand beds as natural nitrate filters: oxygen-depleted zones at the bottom host bacteria that process nitrate. The technique works, but an accidentally deep, undisturbed, organic-rich sand layer is not a deep sand bed — it is a possible source of toxic hydrogen sulfide pockets, which is why beginners are usually told to keep sand shallow or stir it, and to avoid burying waste. Deliberate deep beds belong to experienced marine husbandry; everyone else should stay in the 2–3 inch comfort zone.
Bare bottom: the underrated option
No substrate means nothing traps waste, cleaning is a five-minute siphon, and dosing volumes are exact. The costs are aesthetic and behavioural: some fish (corydoras, loaches) genuinely prefer a soft sand bed to sift, and grazing species miss the surface biofilm. Bare bottom is standard for quarantine and hospital tanks — Aquarium Co-Op's quarantine guide runs bare — and common in goldfish and grow-out setups where waste load is the main enemy. For a display community tank, most keepers compromise on the shallow planted band.
Turning depth into a shopping list
| Setup | Depth | 60 × 30 cm floor needs |
|---|---|---|
| Planted (sloped) | 5–7.5 cm | 9–13.5 L (≈14.4–21.6 kg sand) |
| Fish-only | 2–5 cm | 3.6–9 L (≈5.8–14.4 kg sand) |
| Bare bottom | 0 | 0 L |
The kilograms are approximate bulk-density conversions (sand ≈ 1.6, gravel ≈ 1.5, aquasoil ≈ 0.8 kg/L); the litres are exact geometry. Run your own dimensions in the substrate calculator and buy by the bag that matches.
Sources
- AquariumCalc substrate calculator: 2–3 in planted-depth guidance, kg/L figures (checked 2026-11-16).
- Aquarium Co-Op — substrate for planted aquariums: grain-size trade-offs (checked 2026-11-16).
- Aquarium Co-Op — quarantine the easy way: bare-bottom quarantine practice (checked 2026-11-16).
Heuristic, not gospel
Depth bands are convention, not physics — plants can root in less, and messy tanks can foul in more. Manufacturer litres-per-bag figures beat generic weight estimates, and any substrate inert or not interacts with your water and cycle. When in doubt, start shallower: adding depth later is easy, removing it means tearing the tank down.