Aquarium substrate calculator for plants

Substrate depth is where most planted tank setups go wrong. Too shallow and plant roots have nowhere to anchor; too deep and you’re wasting expensive aqua soil and creating dead spots where mulm accumulates and produces hydrogen sulphide. The right depth depends on what you’re growing.

Carpeting plants — Hemianthus callitrichoides, Glossostigma, Marsilea — need a minimum of 5 cm. Stem plants and mid-ground species are comfortable at 6–8 cm. Large-rooting background plants like Echinodorus and Vallisneria want 8–10 cm. Most scaped tanks use a slope: 4–6 cm at the front, rising to 8–12 cm at the rear, which creates depth perspective and gives the right profile for plant types at each position.

This calculator uses the average of your front and back depths to work out total volume, then converts to kilograms using the actual density of the substrate type you choose. It also handles two-layer systems (pumice or peat base under aqua soil) and outputs a bag count based on the 9 L bags most Australian aquatic retailers stock.

The Maths Behind the Numbers

Volume in cm³ = Tank length × Tank width × Average substrate depth. Divide by 1,000 to convert to litres. Multiply by the substrate’s density (kg/L) to get kilograms.

Example: a 90 × 45 cm tank (standard 4-foot) with 5 cm at the front and 9 cm at the back. Average depth = 7 cm. Volume = 90 × 45 × 7 = 28,350 cm³ = 28.35 L. Using ADA Amazonia aqua soil at ~1.4 kg/L: 28.35 × 1.4 = 39.7 kg. At 9 L per bag, that’s 4 bags — though you’d typically buy 5 to give yourself a cushion. The substrate cost for this one tank would be $160–$200 at current Australian aquatic store pricing.

Why Substrate Density Matters

Different substrates have very different densities, and getting this wrong means you either buy way too much or come up short at a critical moment. Fine silica sand weighs around 1.6 kg per litre. Gravel sits at 1.5–1.8 kg/L depending on grain size. Planted aquarium soils like ADA Amazonia or Seachem Flourite are lighter — around 1.2–1.5 kg/L — because they contain fired clay particles with lots of air space. Pumice, used as a base layer for drainage and gas exchange, runs as low as 0.9–1.1 kg/L.

A two-layer system — pumice or coarse gravel underneath, quality plant substrate on top — is worth considering for tanks over 90 litres. The base layer improves root-zone gas exchange and reduces the amount of expensive aqua soil needed. The calculator’s two-layer mode gives you separate volume and weight figures for each layer so you can shop them independently.

A Practical Planted Tank Setup: 120 cm Tank

A 120 × 50 cm tank with a 3 cm pumice base and 7 cm of ADA Amazonia on top (front 5 cm / back 9 cm average = 7 cm). Pumice: 120 × 50 × 3 = 18,000 cm³ = 18 L → 18 × 1.0 = 18 kg pumice. Amazonia: 120 × 50 × 7 = 42,000 cm³ = 42 L → 42 × 1.4 = 58.8 kg = about 5 bags (9 L) plus one spare. That’s a lot of substrate — and for a tank this size, substrate alone before plants, hardscape, and livestock is a meaningful setup cost.

One thing many beginners discover the hard way: aqua soils like Amazonia release ammonia for several weeks after setup. This is normal and expected; the tank needs to cycle before fish are added. Plan for 4–6 weeks of fishless cycling, especially with a nutrient-rich planted soil.

Getting the Most From Your Substrate

Don’t rinse aqua soil before use — the fine particles and nutrients are part of the product. Do rinse sand and gravel thoroughly until the water runs clear, or you’ll spend days watching a cloudy tank before it settles.

If you’re using a CO2 system (recommended for most planted tanks), set the substrate slope before adding water and CO2. Adjusting the hardscape and slope once the tank is full is genuinely miserable. Get the depth right the first time by measuring from the front glass at several points, not just the corners — tanks aren’t always perfectly level.

Substrate capping (a thin layer of coarse sand over aqua soil) reduces the risk of soil particles being disturbed by water flow or digging fish. A 1–1.5 cm sand cap adds roughly 1.5–2.5 kg per 90 × 45 cm tank — budget for it separately if you’re using this approach.

For help with other tank planning figures, an aquarium volume calculator can give you total water volume for dosing calculations once the substrate is accounted for.

FAQs

How deep should substrate be in a planted aquarium?

Carpeting plants need at least 5 cm of substrate. Most mid-ground and stem plants do well at 6–8 cm. Large background plants with extensive root systems prefer 8–10 cm. In practice, most planted tanks use a front-to-back slope with 4–6 cm at the front glass rising to 8–12 cm at the rear, which provides adequate depth for all plant types and improves the visual perspective of the scape.

How many kg of substrate do I need for a planted tank?

It depends on your tank footprint, desired depth, and substrate density. As a rough guide, a standard 90 × 45 cm tank with a 7 cm average depth needs around 28 litres of substrate. At aqua soil density (1.4 kg/L), that’s approximately 39 kg — usually four to five 9-litre bags. Use this calculator to get a precise figure for your specific tank dimensions and substrate choice.

What is the best substrate for a planted aquarium in Australia?

Nutrient-rich planted soils like ADA Amazonia, Tropica Aquarium Soil, and Seachem Flourite Dark are the most popular choices for heavily planted tanks. They provide a ready source of nutrients for roots and support healthy plant growth without heavy fertilisation initially. For lower-tech setups, an inert substrate like fine sand or gravel with root tabs works adequately and is considerably cheaper.

Can I use normal sand or gravel for a planted tank?

Yes, but inert substrates provide no nutrients, so plants will rely entirely on the water column and root tabs for nutrition. Fine river sand or pool filter sand works well for carpeting plants with shallow roots. Coarser gravel is better for large-rooting background plants. Avoid calcium-rich substrates like crushed coral or aragonite, which raise water hardness and pH — unsuitable for most aquatic plants.

What is a two-layer substrate system for planted tanks?

A two-layer system uses a base layer of porous material (pumice, lava rock, or coarse gravel) underneath the main planted substrate. The base layer improves root-zone gas exchange, reduces compaction, and allows you to use less expensive top substrate. A common setup is 3 cm of pumice base under 6–8 cm of aqua soil. This approach is especially effective in tanks over 90 litres where root-zone health has a larger impact on long-term plant performance.

How do I calculate aquarium substrate in litres?

Multiply tank length × width × average substrate depth, all in centimetres, to get the volume in cubic centimetres. Divide by 1,000 to convert to litres. For a sloped substrate, average the front and back depths before multiplying. This is the volume of substrate you need, not the tank water volume — remember that substrate displaces water, so your actual water volume will be less than the tank’s rated capacity.

Does substrate depth affect water quality in a planted aquarium?

Yes. Too little depth (under 3 cm) limits plant root development and reduces biological filtration in the substrate. Too much depth (over 10–12 cm) creates anaerobic zones where beneficial bacteria can’t survive and sulphide gases accumulate. These gases smell like rotten eggs and are toxic to fish. Keeping substrate depth in the 5–10 cm range and occasionally disturbing the surface with a maintenance vacuum prevents the deep compaction that causes problems.