Practical Fishkeeping Diploma - Week 5, Day 3

Here's the third of a five-part series of lessons to guide new aquarists through the basics of fishkeeping.

Week 5, Day 3: Substrate and decoration

The majority of aquaria, especially those at home, contain some sort of substrate on the base, as well as decorative objects for aquarium inhabitants to interact with, whether that be for cover, spawning, or otherwise.

Benefits of substrate can include:

Creating a natural/calming environment for fish.

Increased biological surface area for filter activity.

Slow release of pH buffering minerals into the water (such as with coral sand).

Rooting medium for plants.

Feeding surface for some species, such as catfish.

Hiding place for some species, such as burrowing catfish or loaches.

Hiding place for eggs and fry, or spawning medium for some species.

Slow release of nutrients for plants.

Drawbacks of substrate can include:

Excess mineral release into softwater (such as with coral sand in extremely acidic tanks).

Deep sand can turn anaerobic without adequate passage of oxygen. This can lead to the development of bacteria that convert nitrate back in to nitrite, or produce highly toxic hydrogen sulphide. 

Substrate can conceal waste like fish faeces and uneaten food, creating the illusion that a tank appears cleaner than it really is.

Incorrect substrate size and texture can damage fish barbels and skin from repeated contact.

Planting substrates may initially contain a source of ammonia which can require them to be sat in the tank without fish for some time.

Substrates may harbour disease causing parasites and pathogens, making them difficult to remove.

Incorrectly sized gravel can get lodged in the mouths of substrate scooping fish.

Substrates may be coarse or fine. Coarse substrates like gravel are easier to clean, but provide a poor medium for plants. Fine substrates like silver sand are great for plant roots, but are more prone to turning anaerobic.

Deeper substrates harbour more waste and are likelier to turn anaerobic than shallow ones. At the maximum, coarse substrates should generally be no deeper than 5cm in a community tank. For fine substrates, no deeper than 2.5cm substrate should be used.  Exceptions may include aquascapes, where deep beds of highly porous planting substrate are used for decorative effect, as well as aquariums with natural filter systems that rely on deep gravel beds.

Artificially coloured gravel is often made from dolomite and then coated in paint and/or resin. Dolomite is a mineral rich substrate and may cause water to become hard and alkaline, making it unsuitable for softwater tanks.

Alternatively, a tank may lack substrate altogether. A bare-based aquarium can be easier to keep clean, as any accumulation of solid waste will be visibly obvious. It can also be easier to extract dormant pathogens on the base in the event of a disease outbreak. This type of system is often used in hospital and quarantine tanks.

Decoration in a tank is both aesthetically pleasing, and beneficial for the fish.

Decoration needs to be considered on a case by case basis. It may act as a boundary marker for territorial fish, or as a hiding place for shy/nervous fish. Fish may require decoration in the form of caves to spawn in or on, or to retreat to at night.

Decoration can be naturally or artificially derived. Examples of aquarium decoration include:

Dried hardwood branches and root structures

Dried, aquarium safe leaves (usually hardwood)

Igneous and sedimentary rocks.

Resin ornaments.

Terracotta or clay tubes and pots.

Decoration needs to be chosen with regard to fish behaviour and water requirements.

Real wood and leaves can leach discolouring tannic acid into the water. To reduce this, wood needs to be soaked for prolonged periods before use. Wood may also need to become waterlogged in order to sink.

Some rocks increase hardness levels. These can be detected by exposing them to acids and checking for a reaction. Usually, the acids required for an observable reaction are more concentrate than the lay aquarist has access to. Rocks from the ocean (such as Tufa rock and Ocean rock) typically contain carbonates.

Some rocks contain dangerous metals, often identified as colourful striations. These can be reactive in acidic tanks, where they increase the acidity levels as they release metals.

Rocks with sharp surfaces may be unsuited to fish that like to rest on decoration, or those that may brush against it (especially highly active fast swimmers).

Cheap artificial decoration may have harmful paints that can be rasped off by suckermouth catfish. Lead paint may be a particular hazard for fish.

Click here for next lesson: Feeding

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Guide to regular fish care

Guide to fish nutrition

Routine aquarium care

Feeding your fish