The substrate is base layer of material in a tank where plants can root and grow. It also a home for bacteria bio-film and microbes which serve as a food source for detritus feeders such as shrimp. Bacteria bio-film also binds small particulate matter together and the substrate serves as a capture point for this. In the substrate, organic debris is broken down further into nutrients that can be uptake by plants.
Substrate can also change water parameters - for example, most aquasoils contain peat that softens the water and reduces the pH. This makes the tank environment more conducive for soft-water fish and plants. Other substrates made of carbonate minerals may raise water hardness instead - Aragonite is often used in aquariums to raise water hardness/alkalinity for Cichlids. Most plants with the except for a couple of species prefer a neutral or slightly acidic substrate.
Substrates can also serve as cosmetic design features; aquascapers need to choose sand to match the rocks used. There is a large variety of sand & gravel colors and textures to choose from. Brown and pale colored sand tend to give a more naturalistic look to a planted tank and are easier to match with hardscape and woods.
Substrate can range from the simple to the exotic. In the tank below I use a mix of raw soil, capped 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.
These are principally composed of sand, gravel or baked clay and remain mostly unchanged over time.
These are made from soil baked into small hard granules, which break down slowly over time.
Hobbyist may make use of a variety of different materials to setup their own substrate layers
Eg. Seachem Flourite, ADA La Plata, CaribSea Eco-complete, Turface, Safe T Sorb
Eg. ADA Amazonia, Fluval Stratum, Tropical aquarium soil, Mr Aqua aquarium soil
E.g. Mineralized top soil, garden soil, earth worm casings, peat,
Common inert substrates include regular aquarium gravel/sand as well as a host of other DIY products: Turface, Safe T-Sorb, Pool filter sand, Black diamond blasting sand. The advantage of baked clay products is that they have better porosity and higher CEC values than say smooth inert silica sand.
Inert substrates are derived from rock minerals or hard baked clay. Inert substrates last forever, and break down extremely slowly if at all. They are the easiest planted aquarium substrates to manage. They all do not contain significant amounts of nutrients (even the ones that market so) and all require fertilization in other forms.
While aquarium soil-based substrates are superior for plant growth, inert substrates are very useful as cosmetic additions to the tank environment and match rocks used in aquascaping well. Most are neutral and do 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 no alter water chemistry - this means the aquarist can isolate their water column parameters easily.
An iwagumi by Jeff Miotke using sand/ gravel.
5 key aspects of inert substrates
In deciding to go with a particular inert substrate, these are five key areas to consider and evaluate:
GRAIN SIZE Grain size - Grain size of about 2mm will work well. Sizes between 1-3mm are generally usable. Avoid planted aquarium substrate sands that are superfine (such as very fine grain silica sand), which compacts more easily. Fine sand is also stirred up too easily by livestock. Pea gravel on the other hand is a bit 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 want to specifically raise your tank's KH values. If you are unsure of an unknown substrate's material - subject it to the acid test to make sure it's not carbonate based.
WEIGHT Very light planted tank substrates are hard to plant in. This is especially so when dealing with plants that are small/delicate or with short roots. This is a small detail, but cannot be ignored.
AESTHETICS Choose sands that match your aquascaping goals. Layered aquariums often require planning ahead; a blackwater tank may look better with a mix of earth toned naturalistic mixed grain sands. A dutch style scape may choose black sand for high contrast against coloured plants. The substrate should also match the hardscape; if you have black/dark rock, white/grey sands match better than brown for example.
CEC VALUE Cation exchange capacity. This reflects the substrate's ability to bind ions, such as fertilizers, keeping them in an available format for plants. This is actually not an important criteria at all, despite some inert substrates being marketed with higher CEC values. Most of the CEC in inert substrates will come from accumulated organic humus, which will build up in inert substrates over time. If you want to really have an impact in this aspect, adding a thin layer of dirt beneath the plain sand/gravel works very well.
This tank utilizes aquasoil in the planted areas and use cosmetic sand for the central path. Carpeting plants growing across the boundary of the two substrates keep them separate.
From an aquascaping perspective, it is important to choose substrate that matches the coloration of the hardscape. White sand matches most rocks easily. However, if you choose a colored substrate such as orange or black sand, matching hardscape becomes more of an issue.
Nutrition from inert substrates
Many commercial planted aquarium substrates may list a long list of chemical elements present in their substrate, however, these elements are mostly locked up in the mineral crystal lattice, and plants cannot get to them easily. That is why inert substrates don't break down significantly over time or change water parameters.
This is similar to listing oxygen as an ingredient in inert silica sand (SiO2) - yes, the oxygen molecule is present, but it's never going to contribute to oxygen levels in the tank because it's locked up in the molecular structure. This means that commercial inert substrates do not innately 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 optimal growth.
Many of these planted tank substrates are still beneficial in that they have good porosity, and come in attractive colours & textures. Some will contain trace amounts of micro-nutrients - better than nothing, but does not replace fertilization in other forms.
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 is made from soil baked into small hard granules. This makes it much easier to manage compared to raw soil, which is easily stirred up. Many aquasoils are spiked with ammonia & other nutrients, which is great for plant growth. However, the soils do break down slowly over long time. The organic matter & porosity also provides a superior bed for bacteria colonization. The grains creates significant pore spacing which prevents compaction and overly anaerobic conditions.
The above shows a close-up of ADA Amazonia's soil granules. Over time the nutrient value of aquasoils will deplete, and need replenishing in the form of root tabs/additional of new soil. However, the benefits of organic matter & porosity will last a long time (years). Plants generally root better in soils than plain sand, many difficult/picky plants grow more stable in aquasoils. Having a rich substrate also means less effort need to balance water column dosing, this offers a tremendous edge in growing difficult species.
Aquasoils aid greatly in plant growth, and hence is the default choice for most planted tank enthusiasts these days.
Ultimately, no matter how rich a planted tank substrate one uses, water column dosing still plays a large part in getting optimal plant growth. The only downside of commercial aquasoils is the cost of attaining them. Different brands also have quite different attributes - some contain a lot more nutrients than others, some are heavier and are easier to plant in.
5 key differences between brands of aquasoil
NUTRIENT CONTENT Ammonia/nutrient content/organic content - Some aquasoils are heavily spiked with ammonia (ADA aquasoil) while others are not (Dennerle). Soils heavily spiked with ammonia need frequent water changes during first couple of weeks and/or pre-cycling the tank for 1-2 weeks before planting.
BUFFERING STRENGTH Most aquasoils contain peat which reduces KH and buffers pH down. Different brands will state different buffering levels. The majority will aim to drop the pH to slightly below 7. This softwater environment is preferred by many plant species.
GRAIN HARDNESS Depending on how they are baked, and their ratio of clay to organics, some aquasoils are harder while some are softer. Hard baked soils can be more brittle. Soft soils may be easier for delicate root systems to penetrate and attach.
WEIGHT Aquasoils with good weight are much easier to plant in. It is harder to anchor plants down in very light aquasoils. (Prodiblo and Fluval tend to be a bit light)
CONSISTENCY This refers to the size of grains, and the amount of other debris present. Some brands are more uniform than others. Small grain sizes are good for planting small delicate foreground plants such as HC or hydropiper. Coarse grains compact less easily over time. Aquasoils with more differentiated grain sizing may look more natural.
Commercial Aquasoil Review
ADA aquasoil is popular among the serious aquascaping crowd. Heavily spiked with ammonia, requires very frequent water changes for the first couple of weeks. Takes 2 - 3 weeks to properly cycle. More troublesome to manage for beginners. We prefer to use a thin layer (as fertilizer) beneath a layer of some other brand (such as Tropica). The other way to apply it is to mix it in small amounts to old depleted aquasoils that have been in the tank for months.
There are few different types of aquasoils in the ADA range. They differ in nutrient content and buffer to slightly different pH levels. If you are growing a planted tank go for the default Amazonia. Amazonia light (not shown here), has lower ammonia content compared to Amazonia. However, we find it more crumbly and uglier in color, so we would advise going for the default Amazonia instead.
Mr Aqua aquarium substrate. Similar to ADA aquasoil, but less rich in ammonia.
Fluval stratum. Minimal leeching of ammonia, very clean soil. May be a little light when planting.
Tropica aquarium soil is another common brand from Europe. Similar to ADA aquasoil, with less uniformed grains but not as ammonia rich. Easy for beginners to handle.
This is a DIY route for folks that want soil based substrates but are unwilling to pay high prices for commercial aquarium substrates.
Garden/top soil is used as a base, with sand as a cap to prevent the stirring up of the soil layer. Plainer, low organic content (10-20% organic content) dirt works better. Avoid soils that are overly clay heavy (if you can mold them into softball sized balls and roll them, probably too much clay content), or add some peat to break up the clay. Avoid compost heavy soils (or use less than an inch).
As raw soil is a non-uniform product, it will be harder to diagnose problems as it is harder to find comparisons - unless you find others that use exactly the same soil you found. Whereas commercial planted tank substrates are widely used and studied, so generally people know how parameters in those tanks will be like. We use this combination often, but it can be hard to manage for new aquarists.
Above: an example of soil layer with sand cap.
My planted tank substrate story
"I experimented with most substrates available on the market over the last 10 years. I typically used two different substrates on each side of the tank just to see whether there was a significant growth difference due to the substrate.
The biggest difference I've 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, more stable in soil.
They root better and have less growth issues compared to using inert substrates. This actually makes things easier for beginners, as soil lessens the need for very tight nutrient control via water column dosing. That being said, there are plenty of tanks out there that grow plants well with inert substrates; especially if the water quality is good and nutrients are provided regularly.
For the few who are confident in managing nutrient cycles and water chemistry, soil can help 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 up-rooting plants."- Dennis
Head here to learn more about substrates or how to create slopes from substrates.