Choosing Aquarium Substrates 101

Posted by Augusta Hosmer on Mar 03, 2023

Read Time: 5 minutes

Dustin's called this the most overlooked part of your aquarium, and it's the truth. Substrates are often the heart of healthy (or unhealthy) fish tank biology. So, in every sense of the phrase, let's dig in.

First Off: Do You Even Need Substrate in Your Aquarium?

Depends on the rest of your setup and what your goals are.

The benefits of aquarium substrate are looks, increased surface area for beneficial bacteria and fungi, more options for plants and animals, and an easier time growing live plants.

That said, you can also grow bacteria on other surfaces and there are plants (like floating species) that can grow without substrate. Some animals, like axolotls (Dustin's absolute favorite) sometimes do better without substrate due to impaction concerns. So it really comes down to what you want and what other surfaces you're providing for biological filtration.

Types of Aquarium Substrate and Pros and Cons for Each

The pet industry is flooded with thousands of aquarium substrate products, but all of those can be broken down into a few main categories:

Aquarium Gravel
Rocks, Marbles, Stones, etc.
Planted Tank Substrates
Sand
Dirt / Soil

Here's a table breaking these down by advantages and disadvantages:

A Table Breaking Down Pros and Cons of Different Aquarium Substrates

For a zoomable version of the above table if your eyes are as bad as mine, here's a link to the Google document.

How Long Does Substrate Last?

Hard question to answer without knowing your specific setup. The short answer is it depends on the substrate type and how heavy your plant load is. A nutrient-rich, planted tank substrate will last longer if plants aren't using a lot of nutrients. Rocks and gravel won't break down or compact as quickly as dirt or sand, but they also don't hold onto nutrients as easily.

It also depends on your fish. Some fish leave your substrate completely alone, while others like cichlids like to root through substrate as part of their natural behavior - which can make keeping your plants and substrate where you want them difficult.

You put WHAT in your fish tank?

Just a disclaimer. I've never done this, so I can't speak on how well it works. I'm not sure I even want to experiment with it because if it goes wrong, the cleanup will be a new circle of hell. But, who knows? Maybe one day curiosity will kill the cat. Or cat litter will kill the cat...maybe I should have chosen a different idiom, here...

Yes, there are people who have successfully used cat litter as an aquarium substrate. The idea is clay is great for aquarium plants and commercial cat litter is typically clay-based.

If you're going to give it a shot, here are a few tips:

Rinse thoroughly and/or try to find a low dust version, because we all know how dusty cat litter can be
It's clay-based, so it won't be inert in water. Keep an eye on your water chemistry, for both positive and negative outcomes (some say it's good to add to filtration because it absorbs so much around it or use it for iron supplementation and plant growth; others talk about how it alters pH and nitrogen-based compounds too unpredictably to be useful).
Only buy the cheap, unscented, untreated litter, especially if you're keeping fish with the plants. Here's a sentence I never thought I'd write: fish do not want Febreze freshness
Cap it with something like gravel; otherwise, you're asking for gallons of sludge. Again, watch out for compaction.

Aquarium Substrate Additives

To make up for some of the cons for the various substrates listed further up, there are also a bunch of things you can add or mix into your substrate. On that note, you can experiment with mixing substrates and substrate additives to see what works best for you.

To give substrates like gravel and sand more nutrients, you can use fertilizers, iron and other supplements, and root tabs.

If you have a chemically inert substrate and are looking for an easy way to alter pH and other chemistry, crushed coral might be an option for you.

A lump of red clay for aquarium plants next to a yellow Expo marker for scale

Clay is also a fantastic additive because it has the finest particle size/pore space, so it holds onto nutrients like crazy. It's also usually rich in iron. Some planted tank substrates, like Fluorite, are clay-based.

Or, you can just add clay around your plants with your existing substrate. The rule for that is 1 pound of red clay/20 gallons of water, and you add it while it's still soft. You can also try something like lazerite underneath gravel, or clay-like, mineral-rich Akadama.

Perlite is also often mixed in with soil or used as filtration.