KNOW-HOW: ALL ABOUT SUBSTRATES
Sand, gravel, soil or enriched clay – what’s the best choice of substrate for your tank and your fish?
From silica sand to aquatic soil, what’s the best type of substrate for your plants and your livestock?
CHOOSING THE right substrate is one of the most important decisions to make when setting up a new aquarium. There are so many options available that it’s easy to end up with the wrong kind or, at least, one that doesn’t work out quite as well as you’d hoped. Replacing substrate is an expensive and time-consuming process, so it’s something you’ll want to get right from the off.
In some cases, the choice of substrate comes down to personal taste, but often, the best one for the job really depends on what fish you’d like to keep or which plants you’re trying to grow.
Leaving the tank bare is a good option for tanks where cleanliness is the number one priority. This includes breeding tanks, hospital tanks and quarantine tanks.
Faeces and decaying organic matter are easily syphoned out during water changes. Medications circulate around the aquarium more effectively, making it harder for pathogens to survive.
Looks unattractive. There’s nothing to shore up rockwork or piles of bogwood, so you need to use self-supporting ornaments and caves, such as flowerpots. There’s nothing for plants to root themselves into, so you’re limited to artificial plants, floating plants, or epiphytic plants attached to bogwood or rocks. Overhead light will reflect upwards off the glass bottom, stressing the fish and causing them to exhibit subdued or washed-out colours to try to blend in.
For decades this was the default choice, usually on top of an under-gravel filter plate, where the gravel acted as the biological medium. While still a popular choice (and rarely a bad one), think about the alternatives before going down this route as plain gravel does have some shortcomings.
Cheap and easily obtained. Lime-free gravel from aquatics shops is chemically inert, so it won’t change the ph or water hardness. Numerous grades available, from fine pea gravel ideal for small fish communities, to almost pebbly gravels less easily disturbed by jumbo species. Its dark colour minimises upwelling light, enhancing the colours of your fish.
Scratches and shortens the whiskers of catfish and loaches as they forage for food. Sharper grades of gravel may cause so much damage that the fish become prone to bacterial infections. While stirring the gravel helps when cleaning out dirt and debris, plant roots will make it harder to do this properly. It’s devoid of mineral nutrients, so provides little benefit to plants beyond anchorage.
These are usually smooth gravels that are painted and then covered with an aquarium-safe varnish. While not to everyone’s taste, they’re often popular with children.
Lots of colours to choose from. Darker gravels, particularly black, can highlight certain fishes’ colours. Chemically inert, so won’t change the ph or hardness of the water.
Unnatural in appearance – and can look a bit naff! Brighter colours, particularly white, reflect light upwards, which can stress fish and cause them to exhibit subdued colouration. Fish with delicate colours look dull next to red, blue and other vivid gravels. Don’t contain mineral nutrients for plants. Pricier than plain gravel.
This lime-free sand is chemically inert and widely used by gardeners. There are two types available – sharp sand and smooth (or silver) sand. Don’t use sharp sand as it will scratch bottom-dwelling fish like loaches and catfish, but silver sand can work very well if cleaned thoroughly before use.
Silver sand is also marketed as pool filter sand, which is usually much cleaner than the smooth sand sold by garden centres.
Inexpensive and easy to obtain. Makes a fabulous substrate for tanks with catfish, loaches, and other burrowing or digging species. Natural in appearance, and a great choice for biotope tanks. Won’t cause a change in the ph or hardness of the water. Set up correctly, deep sand beds become partially anaerobic, fostering the growth of denitrifying bacteria that can lower nitrate levels.
Contains little in the way of mineral nutrients, though plants will root readily. It reflects some light upwards, so some fish will show weaker colours. Big fish can move it about, potentially undermining rockwork. Some sand may end up in the filter if the filter inlet is too close to the substrate. Sand will scratch the tank if caught between the glass and an algae scraper.
Some are artificial, some natural, and some are industrial by-products. Whatever their origins, they look superb in the right tank, offering more variety than plain silica sand.
Usually chemically inert – but check with the manufacturer. Various dark shades are available, including black volcanic sand that can look absolutely fabulous in planted aquaria, showing off the subtly coloured fish and shrimps to best advantage. Cons Dearer than plain silver sand. Some are too sharp, so they aren’t suitable for burrowers like catfish and loaches. Some types contain mineral nutrients that enhance plant growth, but most of them don’t.
We don’t recommend you dig up your garden, but natural soil products from aquarium shops are useful for helping to replicate the substrates found in rainforest streams or pools.
Very naturalistic, especially with some Catappa leaves and bogwood roots added. The dark colour enhances many fish, especially those with fluorescent markings. Tends to tint the water over time, further enhancing colours. The tea-coloured water is particularly appreciated by blackwater habitat species, such as Discus. Burrowing fish love it and behave in a very natural manner –
especially elephantnoses and spiny eels, which don’t thrive in tanks with gravel substrates. Plants root quickly, as you would expect.
Better in tanks with small fish. Larger fish will disturb the soil, making the water murky and potentially clogging the filter inlet. Contains some mineral nutrients, particularly iron, but this varies between brands, so you might need to use additional fertiliser. Quite expensive, although in tanks without rooted plants you only need enough to cover the bottom glass.
LAVA ROCK CHIPPINGS
This is similar to the large pieces of ‘lava rock’ sold as decoration and makes a good alternative to plain gravel.
Lighter than gravel. It has a porous structure, which allows slow movement of water through the substrate. This creates partially anoxic conditions that foster the growth of denitrifying bacteria, and maintenance of non-oxidised mineral ions that plants can use. Open structure, so good for under-gravel filters and tanks with substrate heaters. Rough texture forms a secure base for securing bogwood and rockwork. Dark in colour, so shows off fish well. Some types include minerals for good plant growth, though periodic use of fertilisers may still be needed.
Rough texture, which can cause damage to burrowing fish. More expensive than gravel.
PLANTED AQUARIUM SUBSTRATES
Often based on mineral-enriched clay, which slowly releases beneficial minerals around the roots of your plants. Plants are more fussed about light intensity than anything else though, so if your tank doesn’t have strong lighting, a fancy substrate won’t make much difference.
The best choice for aquariums with rooted plants. Vigorous plant growth is the easiest way to avoid algae problems, so plants that get their nutrients from the substrate, rather than from the water column, will really appreciate this stuff – as will your tank and fish. The better brands are guaranteed to last for five years before needing replacement. Fine and smooth, so good for burrowing fish. Dark brown, so works well against the colours of your fish.
Very expensive, although this is offset by the reduced need for periodic fertiliser dosing. And the better plant growth means that you won’t be replacing your plants every few months!
BELOW: A stark contrast between a naturalistic look and colourfully adventurous.
ABOVE: The wrong substrate can destroy catfish whiskers.