How to Choose an Aquarium Substrate

If you’re new to aquariums, you may have heard the term ‘substrate’ mentioned in some of the articles you’ve read. What this term refers to is the substance that covers the bottom of an aquarium. It can be anything from gravel to potting soil, but it’s one of the most important choices you can make when setting up your aquarium.

Before choosing a substrate, you first have to determine how the tank is going to be used. Will it be a planted tank? A breeding tank? Its intended purpose will have a huge impact on what type of substrate should be used, and some are completely inappropriate for certain types of setups.

Listed below are some of the more popular types of substrates, and the type of tanks they are commonly used in.

Gravel

Gravel is the default choice for most people when they’re setting up their first aquarium. It’s relatively cheap, reliable, and as long as you stay away from neon orange, it looks pretty good. But even for the more experienced aquarist, it’s still a good choice.

Gravel is incredibly versatile and can be used in everything from simple setups, to planted tanks and even breeding tanks. Its irregular surface allows for vast amounts of beneficial bacteria to colonize it, which helps to maintain good water quality.

When choosing gravel, it’s important to focus on both the size, and the color of it. The size of aquarium gravel is pretty standard, but some stores do sell large grain gravel. While large gravel does have its uses in some fish tanks, it’s usually best to stay with the smaller grain size.

The color is a much more important choice than the size of the gravel, and I would always recommend staying away from the more garish colors (affectionately referred to by some as clown puke). Most fish prefer a dark, muted substrate in their aquarium, and there is some evidence that a bright substrate can contribute to stress in a fish.

Gravel needs to be regularly cleaned with a gravel vacuum and it’s important to vacuum deep into the substrate during each water change. If you don’t, debris and waste will begin to build up between the grains of gravel and may start to affect the water quality. I strongly recommend using a Python No Spill Clean and Fill Aquarium Maintenance System (UK Link). It makes water changes a breeze, and does a great job keeping the gravel clean.

Sand

Sand makes for an absolutely stunning substrate in most aquariums, but it can also come with a whole host of problems. It is generally not recommended for someone just starting out, but if you’re willing to take a few precautions, it is a very affordable and attractive substrate.

A sand substrate works best in a simple setup with just fish, though with a bit more effort, it can be used in a planted tank. It is generally not recommended for a breeding tank, or a hospital tank.

One of the biggest benefits of sand is its affordability. Nothing is cheaper than sand for an aquarium, and it’s usually less than a quarter of the price of gravel. With that being said, you have to carefully choose what type of sand you use.

You should only ever use play sand, and it should be thoroughly washed prior to adding it to an aquarium. It can be washed in a clean bucket (make sure the bucket has never held any household cleaners) and it should be rinsed repeatedly until the water runs clear.

And while it’s both cheap and attractive, it does have its problems. The major problem you will face is the sand becoming compacted over time. This makes it very difficult for plants to root in the soil, and you will need to regularly stir up the sand. Adding Malaysian trumpet snails can help with this problem, and if you don’t mind the sight of snails in your aquarium, I recommend adding them to any tank that has a sand substrate.

The other problem you may face is far rarer, and it’s the formation of anaerobic pockets in the sand. If this occurs, and the resulting poisonous gas is released, it can quickly wipe out the inhabitants of your fish tank. However, it is extremely rare, and can be combatted by regularly stirring up the substrate, adding Malaysian trumpet snails, and keeping rooted, live plants. Keeping the substrate depth to a minimum (around 3 inches:7.5 cm) will also help to prevent this from occurring.

A sand substrate is cleaned differently than gravel, and the vacuum hose needs to be held well above the surface of the sand. The goal is to suck the waste off of its surface, without suctioning the sand up too. If a regular cleaning schedule isn’t maintained, visible waste will accumulate on the surface, and it tends to look very unsightly.

Eco-complete

CaribSea Eco-Complete  has become my go to substrate for planted tanks in recent years, and I would highly recommend it to anyone reading this article. However, it really only benefits planted tanks, and its benefits are minimal without live plants

It is rich in all the minerals your plants need, and comes in attractive colors. It is completely natural and is free of any chemicals or dyes. Eco-complete comes seeded with live bacteria, and its highly porous nature provides a huge surface for colonization by beneficial bacteria. This substrate is perfect for live plants, and it encourages thick, luscious growth.

It should never be washed prior to adding it to an aquarium, and washing may remove the beneficial bacteria that it is seeded with. It should be added directly to an aquarium prior to it being filled with water.

Crushed coral is something of a niche substrate and should only be used in aquariums where you are actively trying to raise the pH. It should be avoided in any tank that contains fish which prefer soft water, and may cause problems for an inexperienced aquarist.

It is often used in African cichlid tanks, although it also rarely used in live bearer tanks (since most live bearers acclimatize well to lower pH).

It needs to be regularly cleaned, since its light color allows fish waste and debris to easily show up on its surface. Some people believe it doesn’t fit with the look of a freshwater aquarium, and is more appropriate for saltwater tanks. However, it still is the best choice if you need to raise your pH.

Soil

Soil is only rarely used for a substrate in freshwater aquariums, and even when it is used, it’s more of a novelty than anything else. You can actually purchase specially formulated aquarium soil, but it can be difficult to find.

The problem with soil lies mainly in choosing the right type of soil, and setting it up properly. Once a soil substrate is up and running, it actually needs less maintenance than any of the other substrates mentioned here.

Only plain soil should be used, and even then you run the risk of adding containments to your aquarium. Some soils contain traces of herbicides or insecticides, and can contain other toxic materials. Most potting soils should be avoided, since many contain additives which will create a terrible mess in an aquarium.

Like sand, soil can become anaerobic, and bubbles erupting from it are a sure sign it’s become anaerobic. Also, some soils are incredibly rich in nutrients and can release ammonia and nitrites for months after being added to an aquarium, which will wreak havoc on any fish in there.

If you manage to avoid all of these pitfalls, soil provides the perfect substrate for live plants, and you will see rich and verdant growth. There is no need to supplement a fish tank with CO2 when using a soil substrate, and very little fertilizer is required.

Fewer water changers are also needed, and some people don’t even use a filter with a soil planted aquarium. However, I would recommend one if only for the current it provides. Plus it’s always good to have a backup, in case something goes wrong with the tank.

Bare Bottom

A bare bottom fish tank should only ever be used in very specific situations. Unless you’re setting up a hospital tank, or a breeding tank, a bare bottom tank should be avoided. There are more pitfalls than benefits to this sort of tank setup outside of these specialized situations, though some aquarists do swear by it, if only for ease of cleaning.

One of the main problems with a bare bottom tank is that it provides very little surface for beneficial bacteria to colonize. This can actually make it more difficult to maintain perfect water quality, but regular water changes will deal with this problem.

Keeping a tank with a bare bottom also removes any sort of natural aesthetic, and can stress out aggressive fish. They may spot their reflection in the glass on the bottom, and constantly attempt to run off the interloping ‘fish’.

The main benefit of a bare bottom tank comes from how easy they are to clean. Waste, uneaten food, and debris are deadly in a breeding tank, but with a bare bottom tank it’s easy to keep on top of any buildup of waste. This helps to keep the water in the pristine condition that newborn fry need.

Hospital tanks are kept bare for the same reasons as a breeding tank, and a bare bottom makes it easy to remove any fish waste. It also allows you to suction the bottom easily, which helps in the control the spread of certain illnesses.