Complete planted aquarium substrate guide

What is the substrate, what does it do?

Substrate is the base layer of material in a tank in which plants can root and grow. It is also home to the bacterial biofilm and microbes that act as a food source for detritivores such as shrimp. The bacterial biofilm also binds small particles together and the substrate acts as a trap for this. In the substrate, organic debris is further broken down into nutrients that can be taken up by plants.

The substrate can also alter water parameters - for example, most aquasoils contain peat, which softens the water and lowers the pH. This makes the tank environment more suitable for soft water fish and plants. Other carbonate mineral substrates can increase water hardness instead - aragonite is often used in aquariums to increase water hardness/alkalinity for cichlids. Most plants, with the exception of a few species, prefer a neutral or slightly acidic substrate.

Substrates can also be used as a cosmetic design feature; aquascapers need to choose sand to match the rocks used. There is a wide variety of sand and gravel colours and textures to choose from. Brown and pale coloured sands tend to give a more naturalistic look to a planted tank and are easier to match with hardscape and wood.

Substrates can range from the basic to the exotic. In the tank below, we use a mixture of raw soil topped with Aquarium Aquasoil.

Aquasoils are the default for many planted tanks these days:

Types of aquarium substrates

While planted tank substrates go by a bewildering number of names and choices in the market, they can essentially be categorised into 3 main types.

Inert Substrates

Common inert substrates include standard aquarium gravel/sand and a variety of other DIY products: Turface, Safe T-Sorb, pool filter sand, Black Diamond blasting sand. The advantage of fired clay products is that they have better porosity and higher CEC values than, for example, smooth inert quartz sand.

Inert substrates are derived from rock minerals or hard fired clay. Inert substrates last forever and decompose extremely slowly, if at all. They are the simplest substrates for planted aquariums to manage. They all contain no significant amounts of nutrients (even those that are marketed as such) and all require some form of fertiliser.

While aquarium soil-based substrates are superior for plant growth, inert substrates are very useful as a cosmetic addition to the aquarium environment and match well with rocks used in aquascaping. Most are neutral and will not alter water parameters.

The best thing about inert substrates is that they are easy to manage. Replanting and rescaping is much easier with inert substrates. Because they are chemically inert, they do not alter water chemistry - this means the aquarist can easily isolate their water column parameters.

An iwagumi by Jeff Miotke using sand/ gravel.

5 key aspects of inert substrates

When deciding on a particular inert substrate, there are five key areas to consider and evaluate:

GRAIN SIZE
Grain size - a grain size of around 2mm will work well. Sizes between 1-3mm are generally acceptable. Avoid aquarium substrate sands that are super fine (e.g. very fine silica sand) as they will compact more easily. Fine sand is also too easily disturbed by animals. Pea gravel, on the other hand, is a little too coarse - smaller plants with fine root systems will not root well in it.

MATERIAL
Material type - Most materials are inert; avoid coral sand/limestone chips unless you are specifically trying to raise the KH of your tank. If you are unsure of the material of an unknown substrate - acid test it to make sure it's not carbonate based.

WEIGHT
Very lightweight aquarium substrates are difficult to plant. This is especially true if the plants are small/delicate or have short roots. This is a small detail, but one that cannot be ignored.

AESTHETICS
Choose sands that suit your aquascaping goals. Layered aquariums often require planning ahead; a black water tank may look better with a mix of earthy, naturalistic mixed grain sands. A Dutch-style landscape may choose black sand to provide a strong contrast with coloured plants. The substrate should also match the hardscape; for example, if you have black/dark rock, white/grey sands will work better than brown.

CEC VALUE
Cation Exchange Capacity. This reflects the ability of the substrate to bind ions, such as fertiliser, and hold them in a form that is available to plants. This is not really an important criterion, although some inert substrates are marketed with higher CEC values. Most of the CEC in inert substrates comes from the accumulated organic humus that builds up in inert substrates over time. If you really want to make a difference in this respect, adding a thin layer of soil under the plain sand/gravel works very well.

This tank uses Aquasoil in the planted areas and cosmetic sand for the central path. Carpets of plants grow across the boundary between the two substrates to separate them.

From an aquascaping perspective, it is important to choose a substrate that matches the colour of the hardscape. White sand goes well with most rocks. However, if you choose a coloured substrate such as orange or black sand, matching the hardscape becomes more of an issue.

Nutrition from inert substrates

Many commercial aquarium substrates for plants may list a long list of chemical elements that are present in their substrate, but these elements are mostly trapped in the mineral crystal lattice and are not readily accessible to plants. This is why inert substrates don't break down significantly over time or change water parameters.

It's a bit like listing oxygen as an ingredient in inert silica sand (SiO2) - yes, the oxygen molecule is there, but it will never contribute to the oxygen level in the aquarium because it's locked up in the molecular structure. This means that commercial inert substrates do not naturally contain significant amounts of nutrients for plant growth, despite what the marketing says. Nutrients would need to be added in the form of root tabs / water column dosing for optimum growth. 

Many of these planted aquarium substrates are still beneficial in that they have good porosity and come in attractive colours and textures. Some will contain trace amounts of micronutrients - better than nothing, but will not replace other forms of fertiliser.

Inert Substrate Reviews

Seachem Flourite (above) comes in attractive textures/colours. It has good weight, and is easy to plant in.

CaribSea Eco-complete (above) has good porosity, but is a little light, which makes it difficult to plant in. Despite its name, it does not actually provide significant nutrients for plants.

ADA la plata sand has a mix of naturalistic grains but comes at premium cost - the cost does not reflect its nutrient content, but rather its aesthetic value.

Pool filter sand; cheap, naturalistic - a cost effective choice for many planted tanks

Turface (left) and Safe T Sorb (right) are both baked clay products that boasts good porosity.

Black diamond blasting sand has a shine to it, but is a bit sharp. Dark substrate offers high contrast against coloured plants.

Commercial Aquasoil

Commercial aquasoil is made from soil that has been baked into small hard granules. This makes it much easier to handle than raw soil, which is easy to stir up. Many aquasoils are enriched with ammonia and other nutrients, which is great for plant growth. However, the soils break down slowly over time. The organic matter and porosity also provides an excellent bed for bacterial colonisation. The grains create significant pore space, which prevents compaction and overly anaerobic conditions. 

Above is a close-up of ADA Amazonia's soil granules. Over time, the nutrient value of aquasoils will diminish and need to be replenished in the form of root tabs or the addition of new soil. However, the benefits of organic matter and porosity will last for a long time (years). Plants generally root better in soils than in plain sand, many difficult/picky plants grow more stably in aquasoils. A rich substrate also means less effort is needed to balance the water column dosage, which is a huge advantage when growing difficult species.

Aquasoils are a great aid to plant growth and are now the standard choice for most planted tank enthusiasts.

Ultimately, no matter how rich a planted tank substrate you use, water column dosing still plays a large part in achieving optimum plant growth. The only downside to commercial aquasoils is the cost of obtaining them. Different brands also have quite different characteristics - some contain much more nutrients than others, some are heavier and easier to plant.

5 key differences between brands of aquasoil

NUTRIENT CONTENT
Ammonia/Nutrient Content/Organic Content - Some aquasoils are heavily enriched with ammonia (APT Feast, ADA Aquasoil) while others are not (Dennerle). High ammonia soils require frequent water changes during the first few weeks and/or pre-cycling the tank for 1-2 weeks prior to planting.

BUFFER STRENGTH
Most aquasoils contain peat, which reduces KH and buffers the pH. Different brands will have different buffering levels. Most will aim to keep the pH just below 7. This soft water environment is preferred by many plant species.

GRAIN HARDNESS
Depending on how they are fired and their clay to organic ratio, some aquasoils are harder and some are softer. Hard fired soils can be more brittle. Soft soils may be easier for delicate root systems to penetrate and establish.

WEIGHT
Aquasoils with good weight are much easier to plant in. It is more difficult to anchor plants in very light aquasoils. (Prodiblo and Fluval tend to be a little light).

CONSISTENCY
This refers to the size of the grains and the amount of other debris present. Some brands are more consistent than others. Small grain sizes are good for planting small, delicate foreground plants such as HC or Hydropiper. Coarse grains are less likely to compact over time. Aquasoils with a more varied grain size may look more natural.

Commercial Aquasoil Review

​

ADA aquasoil is popular with the serious aquascaping crowd. Highly enriched with ammonia, it requires very frequent water changes for the first few weeks. Takes 2-3 weeks to cycle properly. More difficult for beginners to manage. 

There are a few different types of aquasoils in the ADA range. They differ in nutrient content and buffer to slightly different pH levels. If you are building a planted tank, go for the standard Amazonia. Amazonia light (not shown here) has a lower ammonia content than Amazonia. However, we find it to be more brittle and ugly in colour, so we would advise going with the standard Amazonia instead.

​Mr Aqua aquarium substrate. Similar to ADA aquasoil, but less rich in ammonia.

Fluval Stratum. Minimal ammonia leaching, very clean soil. May be a little light at planting.

Tropica aquarium soil is another popular brand from Europe. Compared to ADA Aquasoil, it is not as rich in ammonia. Easy to use for beginners.

DIY SUBSTRATES

This is a do-it-yourself solution for people who want a soil-based substrate but are unwilling to pay the high prices of commercial aquarium substrates.

Garden/topsoil is used as the base, with sand as a cap to prevent disturbance of the soil layer. Soil with a low organic content (10-20% organic content) works better. Avoid soil that is too heavy in clay (if you can roll it into softball-sized balls, it is probably too heavy in clay), or add some peat to break up the clay. Avoid compost heavy soils (or use less than an inch).

As raw soil is an inconsistent product, it will be harder to diagnose problems as it is harder to find comparisons - unless you can find others using exactly the same soil you found. On the other hand, commercial growing media are widely used and studied, so people generally know what the parameters will be in these tanks.

We often use this combination, but it can be difficult for new aquarists to manage.

Above: an example of soil layer with sand cap.

Summary

We have experimented with most of the substrates on the market over the last 10 years. We have typically used two different substrates on each side of the tank to see if there is a significant growth difference due to the substrate.

The biggest difference we have found over the years is the difference between inert substrates (Flourite, Eco-complete, Turface etc) and soil (commercial baked soils such as ADA, Controsoil or raw soil - topsoil/dirt/garden soil). Most plants grow better in soil.

They root better and have fewer growth problems compared to using inert substrates. This actually makes things easier for beginners, as soil reduces the need for very tight nutrient control via water column dosing. That said, there are many tanks out there that grow plants well with inert substrates; especially if the water quality is good and nutrients are added regularly.

For those few who are confident in managing nutrient cycles and water chemistry, soil can provide an edge in achieving better quality plant growth. The only downside is that it can be messy - but this is easily countered by additional vacuuming/water changes after uprooting plants.

Head here to learn more about substrates or how to create slopes from substrates.