Purpose and Types of Aquarium Substrate

Aquarium Gravel
Daniel Ahlqvist

With rare exception, aquariums are set up with some sort of substrate covering the tank bottom. The substrate is available in a variety of color and materials, giving aquarium owners a wide range of options when setting up an aquarium.

Because the substrate isn't as easy to change as other decors in the aquarium, it's wise to spend some time choosing the type and color of the substrate before setting up the tank.

Purpose of Substrate

The substrate serves several purposes; some are key to a healthy habitat, while others are merely aesthetic.

1.  First and foremost, the substrate serves a role in the nitrogen cycle by acting as a medium in which beneficial bacteria colonize and grow. Although the substrate is not the only host for these important bacteria, it is where a significant number of them reside. In addition to supporting bacterial colonies, the substrate is also a medium for live plants to take root and draw nutrients. Special substrates are available that provide key nutrients for live plants.

2.  Substrate creates a more natural habitat for the fish, and it's particularly important for fish that like to burrow. Bottom-dwelling species enjoy rooting in the substrate for tidbits of food that have fallen there. The substrate can also make fish feel safer, as it does not reflect images of the other fish in the tank as glass can.

The mottled color of the substrate adds to the feeling of being safe. Some species of fish scatter their eggs on the bottom of the tank. If the tank bottom is bare, the eggs are clearly visible and more likely to be consumed by adult fish. A mottled substrate will help make the eggs less noticeable.

If the substrate is large enough, some eggs will fall in between the open spaces and be protected.

3.  Last, but not least, substrate contributes to the overall aesthetic appeal of the aquarium. When combined with plants, rocks, driftwood, and other decors, it creates a visual landscape that is pleasant to the eye and has a calming effect overall. A well-crafted aquarium is known to have a positive health benefit with those who gaze at it. Choosing a hue of the substrate that compliments your fish can serve to noticeably showcase their colors.

Substrate Material

The substrate is available in a wide range of materials. However, the majority of aquarium owners choose standard gravel that is readily available at pet shops. Gravel comes in a variety of sizes, colors, and even shape. Any gravel that is marked for aquarium use is treated. Next to gravel, sand is one of the other choices often considered. Fish that enjoy burrowing are particularly fond of sand substrate. Another commonly used substrate is crushed coral, which has the effect of raising the pH and increasing the buffering capacity of the water.

This is particularly helpful when keeping certain species of cichlids that prefer harder alkaline water.

Large river rock is sometimes used by itself or with gravel beneath it. The rock is attractive and makes for a natural setting. Marbles, on the other hand, are far from natural. However, they are often used when breeding egg scattering species of fish. The eggs will fall between the marbles and out of reach of the adult fish, who enjoy eating the tasty freshly laid eggs. Marbles are also used in fish bowls, allowing for ease of maintenance. If live plants are kept in the tank, it is common to use laterite or vermiculite as substrate. These materials can store and release important nutrients for the live plants. They are usually used as a lower layer of the substrate and covered with a layer of gravel.

Substrate Quantity

The substrate is generally filled to a depth of approximately one and a half to two inches. Additional depth may be warranted if keeping live plants that produce a robust root system, and need the depth. When using sand, the depth is a bit less, in the range of an inch to an inch and a half. More than that can cause anaerobic zones that are problematical. This is also true of gravel substrate that is filled too deep.

Substrate Color

Color is a topic that often gives rise to heated discussions. Some feel strongly that substrate color should mimic natural habitats, while others choose a substrate color based on personal preference -- bright pink, neon blue, fire engine red or a host of other colors. Clearly, those are not natural colors that fish experience in nature, but it isn't going to directly harm the aquarium inhabitants. If you do a good job of caring for the fish, the color of the substrate is a minor issue. Most fish are quite adaptable, and if kept in a well-maintained tank with proper water conditions and good food, they will do fine regardless of the substrate color.