Aquarium gravel, or substrate, makes the tank more attractive, and it comes in a variety of colors and sizes, from tiny pebbles and sand to large river rocks. However, it also serves several important purposes beyond decoration, though there are some situations in which substrate isn't desirable.
Perhaps the most important function aquarium gravel serves is to provide a home for beneficial bacteria. Those bacterial colonies are necessary to eliminate waste from the fish, leftover food, and plant debris. The bacteria can live without a comfy gravel bed, but they might not grow in sufficient quantities to keep the aquarium safe for your fish.
If the tank is left with a bare bottom, it will be necessary to change the water more often to keep harmful waste from building up. But if the tank is heavily stocked, even frequent water changes might not be enough to keep ammonia and nitrites at bay. In that case, substrate is critical to help maintain a healthy environment.
Substrate is part of creating a pleasant habitat for your fish. It gives the fish—especially those that like to burrow—places to hide, and it provides enrichment for bottom-dwellers that prefer to forage through the substrate for bits of food. Plus, it helps to reduce reflections within the tank that can stress fish.
Substrate can also be used to improve the chemistry of the water. For instance, if your fish require hard water, a coral substrate can help you achieve the correct balance.
Moreover, substrate can provide a safe home for fish eggs. Larger substrates allow eggs to drop out of reach of hungry fish that wouldn't hesitate to eat their own offspring. Likewise, the infusoria, or microscopic organisms, that might be harbored in the gravel bed are a good first food for newly hatched fish.
Home for Live Plants
If you have live plants, a substrate is critical and second only to lighting in keeping your plants alive. The proper substrate in planted tanks can ensure your plants root well and have their nutritional needs met.
Laterite and vermiculite are common substrates for planted tanks, as they store and release nutrients for the plants. They're usually used in combination with gravel. Furthermore, some aquarium plants with more substantial root systems require extra depth to the substrate, so take that into account when assembling your tank.
You may not be able to see all the microscopic ways substrate benefits your fish, but you'll definitely notice how it enhances the aquarium's overall aesthetic appeal. Besides adding a design feature to the tank, substrate hides waste and other materials that otherwise would float through the water. Try having a bare-bottomed tank for even a day, and you'll be amazed at how much "junk" appears on the bottom.
In addition to hiding undesirable materials, substrate shows off what you really want to see: the fish. For instance, a silver fish against bare glass doesn't stand out well. But look at it against a dark substrate, and suddenly all its features pop. Besides making viewing more fun, substrate in a contrasting color to the fish allows you to have a better look at any health issues or odd behavior in your fish.
When Substrate Isn't Ideal
There occasionally are situations in which substrate isn't necessary or even desirable.
The most common circumstance when you wouldn't use substrate is for a grow-out tank to hatch eggs and raise young fry. Grow-out aquariums must be kept scrupulously clean, requiring frequent water changes and prompt vacuuming of waste. Fry are so tiny that it is often difficult to distinguish them against a gravel substrate. Thus, it's easy to accidentally suck them up when vacuuming the substrate or changing the water.
Hospital tanks also often omit substrate, which can harbor pathogens that might linger and infect the next inhabitant. A bare-bottomed tank is a good way to avoid that possibility. For the same reason, some owners also choose to exclude substrate from quarantine tanks.
Finally, some people believe brightly colored gravel can stress fish, possibly because it looks nothing like their natural environment. Stress is a major contributor to a compromised immune system in fish, leaving them susceptible to disease, so you might want to err on the side of caution with a more natural substrate. Paint from the gravel also can leech into the water, so make sure any product you use is completely non-toxic to fish.