The Substance of Substrate

Posted by Aquatropic Staff on September 22, 2023

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The stuff at the bottom of your aquarium can be a bunch of different things: sand, gravel and pebbles are all pretty common in freshwater aquarium keeping. There are others too, and some are vital for those of you intending to keep aquarium plants. As a group, we call these materials “substrate” which Mirriam-Webster defines as “a base on which an organism lives.”

The first question that comes to mind when talking about substrate for your display, isn't “which do I choose,” though this is a good question, which we will delve into shortly. The first question is “do I need a substrate?” Answer: No, you don't. There are upsides to not having one either, substrates can easily get full of fish waste and uneaten food, becoming trap for nutrient pollution in your aquarium. You'll find that people who are breeding fish and raising the fry almost never have substrate in these aquariums unless what they are breeding requires it. Bare bottom tanks are just easier to clean, and much easier to remove fish and fry from when needed.

Why have a substrate then? We are keeping up appearances. On the marine side of the hobby, the bare bottom of an aquarium will end up getting covered with an attractive, multicolored algae called Coralline. Here on the freshwater side, we are not so lucky, and the bottom of a substrate-less tank, just gets covered in biofilm, which, while a good food source for many fish, isn't the most attractive surface ever. So, we add a variety of different bases. Most are just for looks, some are excellent for biological filtration, some will affect the water chemistry, and some can provide nutrients for planted tanks.

Most of us started fish keeping with a small aquarium, maybe even a fishbowl, that was probably a kit. These kits inevitably include pebbles as a substrate. This is a big category, and basically includes anything with a grain size bigger than 5mm or just under a quarter of an inch. (Grain size is a measurement of how big the individual pieces of substrate are.) This category includes things like small, polished stones, glass beads, actual river pebbles, even some plastics and the list goes on. Some of them are very pretty, but because of the grain size, they do not offer a useful surface for biological filtration, and the large gaps between the grains allow for food and fish waste to fall into them easily and are less easily removed. They do have uses but are best suited for under gravel filtration that has a reverse flow so these waste products can't settle there, and that water gets pushed through a canister or the like to make up for the lack of filtration which would be done by the sand in a normal under gravel setup. All in all, we don't recommend pebbles as a substrate, but if you love it, there are ways to make it work.

The next category down is generally called gravel, and this category is a little fluid sizing wise. There isn't a strict industry standard for what qualifies as “gravel” or “sand” (or even “pebbles”). Generally, we look at gravel as things that are between 2mm and 5mm in grain size (.07-.2 inches). Gravels smaller grain size prevents some of the waste products from building up and offers a more suitable environment and surface area for the kinds of bacteria that will help filter out nutrient waste. It is easy to maintain and can also have secondary benefits which we'll get into in a couple paragraphs.

The finest category is sand, and this is pretty much everything that has a smaller grain size than gravel. These make great choices for certain displays and offer an excellent amount of surface area for bacterial activity. The caveat for sand is that some of them come in extremely fine grain sizes and these can easily be blown around by your filter systems, which can irritate the other residents of your aquarium. These very fine sands can also be difficult to clean as they aren't much heavier than the fish waste you will be trying to siphon out. As a result, super fine sands usually look amazing once they settle, but start to look dirty fairly quickly. Very small grain sizes are frequently called “Sugar Sand” or “Oolitic Sand.” Unless you are planning to keep a fish or invert that requires this fine of a substrate, we recommend staying away from these.

Sand and Gravel are both available in a variety of materials, most of which are neutral (or inert – meaning they won't affect the water quality of the aquarium they are placed in). There are some substrates which can affect water parameters and coloration. Some substrates like crushed coral and aragonite sand can act as a buffer in your aquarium helping to keep pH elevated for fish and inverts that like these environments. For those of you trying to have an acidic water profile, choosing an aragonite-based substrate will make your life difficult.

Soils are a category completely unto themselves. Soil substrates are a requirement for those of you who are interested in having more than a few very hardy plants. The soils topic, and how to use them in conjunction with other substrates is a topic beyond the scope of this article, and something we will do a deep dive into in the coming weeks. If you decide you need to add soils to your tank, also check out our primer on Carbon Dioxide Dosing and Planted Tank Lighting as these three concepts are bound together. As we discussed earlier, outside of planted tanks, much of the decision about your substrate choice is dependent on aesthetics. If you love the look of pure white sand, it's your tank so go for it. If you are looking to make an accurate representation of a certain habitat, then your choice of substrate will obviously be impacted by this. As a general rule, fish will display their best colors when surrounded by darker colors. This partially explains the popularity of black and brown substrates (which do look cool, we're fans.)

How much substrate do you need? Here we say less is more. Overly deep beds of substrate are difficult to clean and can get anoxic (or depleted of oxygen) if deeper than a few inches deep. This isn't usually a problem unless disturbed, when it can be very stinky and perhaps even toxic for your fish. Unless you have fish or invertebrates that require deep sand to burrow in, live in, or move around, we suggest having less than an inch or two of depth throughout the aquarium.

We hope this helps you as you start to plan your next aquarium. If that next aquarium is going to be planted, check back here for our article on substrate for planted displays. As a cautionary tale, don't use sandbox sand, or rocks from the yard in your aquarium. Some of these sands contain huge amounts of silica and trace amounts of fertilizers or pesticides/herbicides; all of which will cause serious problems for your aquarium, or even kill things in it. Instead, go and talk to your Local Fish Store about the substrates they use and carry and what they recommend. Spending a little money up front on things like this, can save you a lot of money and heartache later.