How to Pick the Best Substrate for a Planted Aquarium

Welcome back to Part 3 of our Getting Started with Aquarium Plants series. In today’s article, we dive deeper into the topic of planted tank substrates. Substrate is the ground covering or “soil” at the bottom of the aquarium that many live plants need in order to grow roots and absorb nutrients. Interestingly, some aquarium plants (e.g., rhizome plants, floating plants, and most stem plants) prefer to absorb nutrients directly from the water, whereas others (e.g., sword plants, vallisneria, cryptocorynes, and certain carpeting plants) mostly feed from their roots. Therefore, the kind of plants you want to keep should affect your substrate choice.

Companies have spent a lot of time and research into developing plant-specific substrates to help plants grow well, but which kind is the best? This article provides a high-level overview of substrates so that you can customize them for your needs, so let’s start by talking about the two main types: nutrient-rich and inert substrates.

Nutrient-Rich Substrates

Before the hobby of planted tanks and aquascaping became more well-known, people took a cue from mother nature and used soil to grow plants. Organic soil contains many essential nutrients for plants, and the texture closely matches the lake bottoms or riverbanks where plants are found in the wild. But what do you get when you mix dirt with water? A big muddy mess. Most people fix this by capping or sealing the dirt under a layer of gravel or sand to prevent the dirt from clouding the water, which works okay as long as you never move any of the plants. Also, soils eventually become depleted of nutrients (as it does with farming), which means the substrate must be reinvigorated somehow. You can either pull out the plants and let the “land” lay fallow while the fish waste reintroduces nutrients or you can remineralize the soil with root tabs and other fertilizers, but both methods tend to cause very murky water that is difficult to clear up.

Easy Root Tabs are made of nutrient-rich topsoil and clay to help grow plants that are heavy root feeders.

Because of the difficulties that come with maintaining dirted tanks, manufacturers created specialized plant substrates such as ADA Aqua Soil and Aquavitro Aquasolum. These compact, nutrient-rich balls of soil are also known as “active substrates” because they tend to lower pH and soften water hardness, so many people use them in crystal shrimp tanks and aquariums with heavy root-feeding plants. However, given that the substrates are primarily made of organic materials, they break down over time and become very muddy like regular dirt. After one to two years of usage, these substrates also become exhausted of nutrients and will need to be remineralized like dirted tanks. Finally, nutrient-rich substrates are usually the most expensive option on the market, so if you are using plants that don’t primarily feed from their roots, there are more cost-effective alternatives.

Nutrient-rich substates are commonly used in crystal shrimp tanks and planted aquariums with heavy root feeders, but they must be remineralized with new nutrients on a regular basis and tend to break down over time.

Inert Substrates

Unlike nutrient-rich substrates, inert substrates come with very few nutrients, which may sound bad at first but keep reading. For example, if you set up your first tank with rainbow gravel from the pet store but later on decide you want to add plants, it will work just fine for most stem, floating, and rhizome plants because they mainly feed from the water column. Just regularly dose an all-in-one liquid fertilizer that contains most of the macronutrients and micronutrients your plants need. If you decide to add a heavy root feeder like an Amazon sword, simply insert root tabs to convert your inert substrate into a nutrient-rich substrate.

Rhizome, floating, and stem plants primarily absorb nutrients directly from the water column, so keep them well-fed with a comprehensive fertilizer like Easy Green.

There are several brands of inert substrates created for planted tanks, such as CaribSea Eco-Complete and Seachem Flourite. Like aquarium gravel, they do not tend to break down over time and therefore do not need to be replaced over time. Unlike regular aquarium gravel, these substrates are made of volcanic or clay-based gravel that usually have a higher cation exchange capacity (CEC). This simply means the materials are better at holding onto nutrients (such as from fish waste or fertilizers) so that plants can easily use them for greater growth. Plus, as inert materials, they do not impact the pH, water hardness, or other water parameters in any significant amount.

While almost any substrate material can be used to grow aquarium plants, remember to avoid the extremes when it comes to substrate size. Very fine sand is hard on plants because the particles are very small and tend to compact together, making it difficult for the roots to easily penetrate and spread through them. Coarse sand, however, creates small pockets between the particles and works much better as a planted tank substrate. If you go to the other extreme and use large river stones as your ground cover, there’s too much empty space between the substrate pieces, which makes it hard for rooted plants to grab onto and get well-established.

Regular gravel works well even with Amazon swords and other root-feeding plants, as long as you keep the substrate fertilized with root tabs.

Which Substrate is Best?

Unfortunately, there is no one right answer. You cannot just look at an awesome aquascape and copy the substrate it uses because everyone’s water is slightly different. For example, in the world of gardening, serious hobbyists test their soil to find out what nutrients they have and which ones are missing. Based on the results, you may need to amend your soil by adding dolomite, peat, or other potting media. In the same way, if you live in a region with soft water and then use ADA Aqua Soil that further softens your water, your plants may be lacking key nutrients such as calcium, magnesium, and manganese. In order to compensate, your optimal substrate choice may actually be a mixture of Aqua Soil and Seachem Gray Coast, an aragonite-based substrate high in those missing ingredients. Therefore, talk to other local planted tank enthusiasts who have similar water composition, and try different substrates and substrate mixes to find out what works best for you.

Very few plants in this beautiful aquascape require substrate, so a cheap, natural-looking sand was used to cover the tank bottom.

Another key takeaway is that spending a ton of money on the most expensive substrate won’t automatically get you amazing results. Instead, be strategic about which plants you’re going to be using and what they specifically need. If you’re buying mostly anubias and only have one heavy root-feeding plant in the corner, save your money by mineralizing the substrate right around it and then fill in the rest of the tank with a cheaper option like gravel. If you’re making a planted tank for African cichlids, the last thing you want to do is lower the pH and soften the water, so don’t pick nutrient-rich substrates if possible.

Hopefully, this article has given you a good overview on planted tank substrates and which types are most suited for your particular needs.