Choosing the right substrate is an important part of setting up an aquarium. Different types of substrates serve different purposes and they each have their own advantages and disadvantages. Most substrates provide a surface for nitrifying bacteria to grow on, keeping your aquarium cycled and healthy. It's important to have a layer of substrate on the bottom as glass bottoms can reflect light and irritate or disorientate fish. It also helps make your aquarium look more natural and can provide a surface for aquatic plants to be planted in or root on to.
 
Substrates can be broken down into two main categories:
  1. Inert Substrates - Sand, Gravel
  2. Active Substrates - Aquatic Soil, Organic Potting Soil, Aragonite Sand

Inert Aquarium Substrates

Inert substrates comprise of rock minerals or hard-baked clay and essentially last forever since they do not break down, or break down extremely slowly. They do not contain any nutrients so you will generally have to supplement nutrients by using Root Tabs if you have plants that are heavy root-feeders, such as Crypt plants and Sword plants. Inert substrates are generally less expensive than active substrates and do not need to be replaced since they do not break down easily, thus, they are great for beginners!

1. Gravel

Gravel Substrate
Gravel is one of the most common aquarium substrates as it is relatively cheap and easy to find at any fish store. It's recommended that you get gravel between 1-3mm in size as larger aquarium gravel will have higher porosity (total space between the individual rock pieces) and tends to trap leftover food/poo, which may cause ammonia spikes if not vacuumed out regularly. It also is not the best substrate to plant plants in as Root Tabs used might leech out nutrients into the water due which again might cause an ammonia spike. Imagitarium Aquarium Gravel or Carib Sea River Sand are some examples of gravel I would recommend using in your aquarium.
 Pros Cons
  • Relatively inexpensive
  • Comes in many colors, sizes, and shapes
  • Can be easily found online/in-store
  • Excess food/waste gets trapped easily
  • Not the most optimal for growing plants
  • Not suitable for certain types of aquatic pets (e.g. Goldfish, Axolotls)

2. Sand

White Pool Filter Sand
 
Sand is another inert substrate that I personally prefer using as it's one of the cheapest substrates you can use by buying non-aquarium specific sand in bulk such as Pool Filter Sand or Diamond Blasting Sand. You can technically also use Play Sand but I have tried it and despite being one of the cheapest type of sand in bulk per lb, you'll have to rinse it A LOT before being able to use it without it clouding the water too much, even then, it might still cloud the water, hence why I personally wouldn't recommend it. which  I also personally love Petco's Imagitarium Black Aquarium Sand as I feel that black sand hides waste at the bottom better and has more contrast against plants. One big plus about using sand is it's heavier than most aquatic soils and makes it easier to plant plants in. It also is the easiest to use Root Tabs in as it is heavy and helps keep the tabs from popping up or leaching into the water. In my experience plants also grow larger and denser roots in Sand compared to Gravel.
 Pros Cons
  • Inexpensive if buying non-aquatic sand in bulk. Great for budget builds!
  • Easiest to plant plants in
  • Safe for most aquatic animals
  • You'll need to supplement nutrients with Root Tabs

Active Aquarium Substrates

Active Substrates come in different forms and can do different things like raise or lower pH by buffering it or contain nutrients such as Ammonia or organic content in the soil granules which help encourage plant growth. Many of these soils are also porous, which help encourage nitrifying bacteria to colonize more surface area, which is essential when establishing your cycle.

1. Aquatic Soils

Ultum Nature Controsoil
 
There are many types of aquatic soils, and they all have their own unique properties. Carib Sea Eco-Complete, for example, contains major and minor trace elements and contains live Heterotrophic bacteria to help break down fish waste, and apparently helps you cycle your tank faster. Another popular aquatic soil is Seachem Flourite, which is porous clay gravel that provides essential nutrients for your plants without affecting your ph. ADA Amazonia uses a rare Japanese black soil and contains organic material to promote healthy plant grow while buffering your water naturally and makes the pH of the water slightly acidic.
Fluval Plant and Shrimp Stratum
Shrimp specific aquatic soils such as Fluval Plant and Shrimp Stratum, which is made out of made of mineral-rich volcanic soil, and is a popular substrate used by Cardinia Shrimp keepers, as it helps buffer and lower pH to around 6-7, which is the recommended pH to keep Cardinia Shrimp in. Other popular brands include UP Aqua Sand, Ultum Nature Controsoil, and Mr Aqua Plant Soil.
 Pros Cons
  • Contains nutrients so it reduces the need to use additional fertilizers
  • Buffers pH (great if you want to lower your pH)
  • Is usually porous, which encourages nitrifying bacteria to colonize the substrate.
  • More expensive than inert substrates such as Sand/Gravel.
  • Needs to be replaced eventually as it breaks down after a period of time.
  • Lightweight, so may not hold plants down easily.

2. Organic Potting Soils

Miracle Gro's Organic Potting Soil Capped with Fine White Gravel
 
Organic potting soils, such as Miracle Gro's Organic Potting Soil, are non-aquatic soils recommended by Diana Walstad in her book, Ecology of the Planted Aquarium, and is most commonly used by aquarists when attempting "Walstad Method" aquariums. It contains organic material that provides nutrients for plants to grow. I recommended that you shift through the soil to remove any large pieces that may float to the surface if the soil layer is disturbed. It is also essential that you cap the top of the organic potting soil with a layer of gravel or sand to keep the soil down. If you are using organic potting soil, you will need to ensure your aquarium is cycled before adding any livestock as it will release large amounts of Ammonia when you first put it into the aquarium.
 Pros Cons
  • Contains nutrients so it reduces the need to use additional fertilizers
  • Much cheaper than aquatic-specific soils
  • Needs to be capped with a heavier substrate such as sand or gravel
  • Can cause Ammonia spikes when first used
  • Hard to re-scape your tank as the soil might be released while unrooting plants and cause an Ammonia spike.

3. Aragonite Sand

Aragonite Sand
Aragonite sand, like Carib Sea Aragonite Aquarium Sand, is popular in Cichlid tanks or reef tanks as it naturally buffers pH to 8.2 as Calcium from crushed coral and shells (which are made of Calcium Carbonate) slowly dissolves into the water, increasing the kH (carbonate hardness), gH (general hardness), and pH.
 Pros Cons
  • Helps raise pH naturally
  • Is porous so it encourages nitrifying bacteria to colonize its surfaces
  • Not really suitable for plants, doesn't contain any nutrients and plants don't grow well in high pH

Conclusion

Honestly, I personally prefer using a Sand substrate, such as Pool Filter Sand or Imagitarium Black Aquarium Sand (which is what I use in majority of my aquariums) and supplementing nutrients using Root Tabs. However, if you can afford it and are experienced with planted aquariums, I would highly recommend investing in an active substrate such as Carib Sea Eco-Complete or ADA Amazonia.
 
 
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