Ask the WWM Crew
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The bottom of the tank is one of the most neglected parts of the aquarium, and as far as many aquarists are concerned there really isn't much to consider. In a freshwater tank, you use gravel, and in a marine tank coral sand. If the tank has an undergravel filter, you'll need a fair depth of the stuff, but if it's just a decorative covering to hide the floor of the tank, then you only need enough to hide the glass. So is that really all you need to know about aquarium substrates? Definitely not! Choosing an unusual substrate is a great way to give a tank a distinctive look, and more importantly there are many types of fish that appreciate specific types of substrate. Gravel and sand
Ordinary aquarium gravel consists of small chips of granite, basalt, and other non-calcareous stones. There are several reasons why it has become the standard substrate for most aquaria, but the most important is that it is chemically inert and so does not affect the hardness or pH of the water. This means that it can be used in soft water aquaria without any risk of it slowly dissolving into the water and thereby raising its alkalinity. Another important factor is that gravel is relatively coarse and irregularly shaped, making it an excellent filter medium when used as part of an undergravel filtration system (a rather misleading name perhaps, given that the filtration is in, not under, the gravel!). Compared with canister filters, cleaning undergravel filters couldn't be easier -- every month or two rake the gravel with a stick and siphon up the muck that comes out.
But gravel has some shortcomings, the most important of which is that it isn't the ideal substrate for fish that need to dig or burrow. In the wild many catfish, such as Corydoras, naturally, use their sensitive whiskers to find food in the mud at the bottom of the rivers and lakes that they inhabit. When kept in tanks with a gravel substrate, they try to do the same thing, but the coarse gravel particles invariably abrade the whiskers to some extent. Whether or not this actually does the catfish any harm is debatable, but there's no question that gravel is positively harmful to fish like spiny eels that actually dig into the substrate. Spiny eels are sensitive fish at the best of times, and when kept in tanks with a gravel substrate they can scratch themselves while digging, and these wounds commonly lean to mysterious but often fatal skin infections.
So with fish like catfish and spiny eels -- not to mention loaches, Mormyrids, gobies, earth-eating cichlids, and freshwater flatfish -- you really want to keep them in a tank with a softer substrate than gravel. Sand is an easy to use option, but aquarists do need to bear in mind that there are at least three different types they are likely to encounter. Each has its uses, but because of their very different chemical properties they are not all equally suitable for any given aquarium.
The most easily obtained aquarium sand is coral sand, which as its name suggests is made up of finely ground coral, along with tiny pieces of things like seashells, calcareous algae, and sea urchin spines. Being made up largely of calcium carbonate, coral sand is a very effective buffering agent, increasing the pH and hardness levels of any aquarium it is used in. For this reason, coral sand cannot be used in any tank with fish that do not like hard, alkaline water, which includes the vast majority of community fish from Asia, Africa, and South America, such as angelfish, tetras, barbs, catfish, gouramis, and loaches. On the other hand, those fish that do appreciate such conditions, such as Rift Valley cichlids, many livebearers, and brackish water fish positively thrive in tanks with coral sand.
Coral sand is relatively coarse, and so to a certain extent can be combined with other types of substrate to create an undergravel filter. A thin layer or coral sand (1-2 cm) on top of a plastic gravel tidy will not impede the flow of water to any great extend, and underneath the gravel tidy can be placed crushed shells or a coarse calcareous substrate such as Calcium Plus. These will all work together nicely to provide an excellent biological filter as well as maintaining the high pH and alkalinity. One issue with coral sand, and indeed many of the other white or highly reflective substrates, is that they often have the undesirable side effect of causing the fish to fade their colours. Not all fish do this of course (hardly any marines do, for example) but the effect can be dramatic with some cichlids and brackish water fish. Mixing in darker sand with the coral sand can help, as will a layer of algae over the sand once the tank matures, but the best solution is to use the rocks, plants, and lighting in the aquarium to create distinct regions of light and dark so that the fish do not feel continually 'under the spotlight'.
An alternative to coral sand is river sand, sometimes called beach sand. The constituents of this sand vary from place to place, but typically it contains some chemically inert materials (such as granite and quartz) along with a proportion soluble calcareous material. So while it isn't such an effective pH and hardness buffer as coral sand, it will still have some effect, so can't be used with soft water fish. Like coral sand, the buffering effect of river sand makes it a good choice with fish that like alkaline water conditions, and it looks especially good with native marines, Central American cichlids, brackish water fish, and livebearers.
On the other hand, river sand cannot be used with an undergravel filter; unlike coral sand the grains are so small and compact so readily that it cannot be separated from a gravel filter bed without impeding the flow of water. River sand can only be used as a decorative medium for covering the bottom of the tank, and in an unplanted tank the depth should be no greater than that which the fish can easily move around while they're digging. The danger comes from food or other pieces of organic matter getting buried in the sand and allowed to decay anaerobically. Anaerobic decay is bad because it produces toxic gases that can leak into the aquarium stressing, and potentially killing, the fish. It is usually recommended that substrates that are not part of an undergravel filter should be no deeper than around 1 to 2 cm. Planted tanks can get away with greater depths, because the roots themselves will oxygenate the substrate preventing the risk of anaerobic decay, and where you are keeping big fish, such as violet gobies or flounders, that can and do shift large quantities of sand, you can increase the depth of sand used according to the size of the fish in question.
The third type of sand aquarists are likely to encounter is silica sand, often called silver sand on account of its shiny, almost metallic, colour. As its name implies, silica sand is made up of silicates such as quartz. While chemically similar to glass, the 'smooth' grade of silica sand is completely harmless to fish. In fact silica sand is especially appreciated by fish that take mouthfuls of sand when they feed, such as the Eartheater cichlids and Awaous gobies. It is also ideal for use with any fish that like to burrow, such as flounders and spiny eels.
Finding silica sand can be tricky because few aquarium stores actually sell it. Instead find a garden centre or a pet store that deals with reptiles. Garden centres sell the stuff for use with indoor plants and as a dressing for potting compost, but do make sure you avoid the 'sharp' grade of silica sand, which is unsuitable for use in aquaria. Reptile stores sell smooth silica sand for use with things like lizards and snakes that need a soft substrate for digging into, especially when laying their eggs. Not all the sand sold in reptile stores is silica sand though: there are several sands produced that include a mixture of sand types including calcareous sand that work as dietary supplements.
I like silica sand a great deal. Not only do the fish love it, but with the appropriate use of a supplement like laterite or aquarium soil it can also make an outstanding medium for aquatic plants. But is it the perfect substrate? I think not; while undoubtedly an versatile and inexpensive substrate it does have some major shortcomings. Just as with river sand, it cannot be used with an undergravel filter, and may also allow for potentially dangerous anaerobic decay. Silica sand is also very reflective, and as noted earlier, some fish fade their colours when kept over such as substrate.
But by far the most contentious issue with silica sand is whether there is a connection between the use silica sand and the growth of diatoms (better known to aquarists as the brown, slimy algae that coat the glass sides of the tank). Since silica sand is made up of quartz grains, essentially the same stuff as glass, the rate at which silicates would dissolve into the water should be trivially small. After all, no one is suggesting that we stop using glass tanks because the cause diatom blooms! Even so, some aquarists have noted blooms of diatoms in tanks after they installed a silica sand substrate, and insist that silica sand is the causative factor. Other aquarists disagree, and given that I've used silica sand many times and never noticed a diatom bloom, I'm minded to agree with them. Certainly factors like high nitrate/phosphate levels and inappropriate lighting are going to be far more important in promoting the growth of diatoms that whether or not you use silica sand. Moreover, even if there is some dissolution of silicates into the water, this won't affect things like hair algae or blue-green algae at all because silicon is not an important nutrient for these organisms.
Peat and soil
Although aquarists have used peat in filters for years, primarily as a tool for acidifying and softening water, but its use as an aquarium substrate is relatively uncommon. This is a shame, because in some situations it can be an excellent choice, lending a very distinctive 'jungle stream' look to an aquarium. It looks especially good with species that naturally inhabit deep and dark forest streams and pools, in particular tetras, dwarf cichlids such as Apistogramma, labyrinth fish, and some types of killifish. These fish not only appreciate the acidity that the peat lends to the water, but they also show off their colours most brilliantly when kept in tanks with a dark brown or black substrate. The blackwater tint that the peat gives to the water also helps. Whereas neon, cardinal, and Glowlight tetras can look a bit subdued in the average tank, kept in dimly lit tanks with a dark substrate and tea-coloured water their natural colours really show through in a most amazing way, especially when kept in large groups. The popular killifish from the general Aphyosemion and Nothobranchus also shine more brightly in dark tanks, and will also use the peat as a place to lay their eggs, mimicking their use of leaf litter and mud in the wild. But be careful with these fish, because they do not like very acidic pH levels, so a buffering agent may need to be used to stabilise the pH around 6 or 7, depending on the species.
Peat is also a nice substrate for fish that like to dig, and fish such as Mormyrids and Synodontis will not only happily dig for food in a moderately deep layer of peat but they will also keep it clean and constantly turned over, removing the risk of uneaten food polluting the tank. While peat is unlikely to become compacted enough to produce pocket of anaerobic decay in the same way as sand, uneaten food can still be a source of unwanted nitrates and phosphates that can promote the growth of algae. It is also possible to use peat with an undergravel filter by separating the peat from a regular gravel bed using a gravel tidy.
There are of course limitations to the use of peat as an aquarium substrate. Compared with sand and gravel, aquarium peat is relatively expensive. You can use peat from a garden centre, but you do need to be very careful about picking the right kind: what you want is sphagnum moss peat that has had no additives or fertilisers added. An alternative made from coconut fibre can also be used. It is usually sold for use with reptiles and amphibians and sometimes comes as solid blocks that can be soaked to produce a remarkably large amount of fibre. Although it works well as far as a useful soft substrate goes, it doesn't affect the water chemistry as much as real peat, which may of may not be a good thing depending on what fish you are keeping.
Price aside, the biggest problem with peat is that it effects to water chemistry profoundly. Soft, acidic water is fine for many aquarium fish, but things like livebearers and many kinds of cichlid do not like such conditions. Even fish that naturally inhabit soft, acid waters may well be acclimated to hard, alkaline conditions at the wholesaler and then the retailer long before you buy them, so carefully adapting them back to soft water conditions is essential. Few fish tolerate sudden changes in water chemistry well.
Another problem with peat is that it is poor in nutrients, and used without any supplements does not make a good medium for growing plants. When peat is used by gardeners it is invariably supplemented with compost or some other, nutrient rich, substrate except when a nutrient poor substrate is specifically needed, as with carnivorous bog plants like Venus fly traps. For good plant growth you really need something that is rich in minerals like iron but low in nutrients like phosphate and nitrate that will promote the growth of algae. There are two popular substrates that fulfill these criteria: laterite and pond soil.
The laterite sold to aquarists is essentially an iron-rich earth similar to the red soils common in the tropics. It is used as a supplement rather than an alternative to a traditional gravel substrate and provides a significant boost to the growth of virtually all aquarium plants. Because it is a fine powder it cannot be used with undergravel filters (it would simply clog up the filter inhibiting the flow of water). Of course building a deep, dense substrate may be good for the plants but there is a risk of anaerobic decay or uneaten food or fish wastes that get buried in the gravel. While the roots of the plants will carry some oxygen into the substrate and thereby reduce the chances of anaerobic pockets developing, the best solution is to use an under tank or in-gravel heater to form convection currents at the bottom of the tank. These will slowly pull oxygenated water into the substrate at a rate sufficient to prevent anaerobic conditions. The key thing is that in contrast with undergravel filters, which move oxygenated water through the gravel very quickly, these gentle currents do not oxidise the mineral ions in the substrate rapidly enough to interfere with their uptake by the plants.
Aquatic, or pond soil, is a special compost produced primarily for outdoor use but which can work very well in aquaria. It lacks nitrates and phosphates, unlike traditional soils and composts, so won't promote the growth of algae. On the other hand, it is rich enough to provide the minerals like iron and copper than plants need to grow well. The main worry most people have with using soil in their aquarium is that they imagine it will be messy and cloud the water, but the soils sold for use in ponds won't do this. While some soil particles and fibres might float to the surface, there should be nothing that the filter can't remove after a day or two. I find creating a bed of pond soil 3-6 cm deep topped with a layer of silica sand 3-4 cm deep works very well, and though there is some mixing of the layers, the plant roots actually stabilise the substrate very effectively (exactly as they do in the wild). The principal downside to using pond soil is the same as that with laterite, namely that it cannot be used with an undergravel filter.
Do you need a substrate at all?
There are a couple of situations where you might want to avoid using a substrate altogether. These are in the breeding tank and the quarantine tank. In some breeding tanks you will need a substrate (for example with killifish that deposit their eggs amongst peat fibres) but in many cases a tank without a substrate works much better. Leaving out the substrate makes it much easier to remove uneaten food and fish wastes using a siphon or pipette.
Likewise, a quarantine tank is a place that you will want to keep as clean as possible. It's debatable whether leaving out the substrate completely is a good idea, after all many fish find a shiny glass tank bottom rather stressful, but even if you do use a substrate it is going to be a very thin, easy to clean layer of sand or gravel rather than anything more substantial.
Getting to the bottom line
There are clearly many more different types of substrate than plain gravel and coral sand, and each lends the aquarium very specific advantages. An aquarist wanting to keep a school of Congo tetras, some Synodontis, and a few Mormyrids and Ctenopoma will find that a substrate of peat will work wonderfully with some large pieces of bogwood and a few epiphytic plants like Anubias nana. The resulting tank will be dark, mysterious, and very atmospheric.
On the other hand, someone who wants to create a vibrantly coloured planted tank will definitely want to consider using gravel or sand supplemented with laterite or aquatic soil. Doing without an undergravel filter may cause some problems, but the improved plant growth definitely makes it worthwhile if that's where your interest lies.
So when it comes to planning your next aquarium, check out the alternatives to plain old gravel available at your local garden centre and reptile store as well as in your local tropical fish shop. You may have more options that you thought!
Substrate: No substrate
Advantages: Easy to clean
Disadvantages: Most fish don't like bright, shiny glass beneath them
Best with: Baby fish, fish in quarantine or treatment tanks
Substrate: Plain gravel
Advantages: Suitable for undergravel filter, chemically inert, easy to clean
Disadvantages: Most fish cannot dig or burrow, not typical of many fish habitats
Best with: Fish that swim away from the substrate, e.g., barbs and tetras
Substrate: Coral sand
Advantages: Effectively buffers water to high pH and hardness levels, works well with digging fish, easy to obtain, can be used with undergravel filters
Disadvantages: Cannot be used with fish needing soft/neutral water conditions, high reflectivity may wash out the colours on some fish
Best with: Marine fish, Rift Valley cichlids, brackish water fish
Substrate: River/beach sand
Advantages: Looks very realistic, works well with digging fish
Disadvantages: Tends to increase pH and hardness, so not good with fish needing soft/neutral water conditions, cannot be used with undergravel filters
Best with: Rift Valley cichlids, brackish water fish, fish that likely moderately hard water such as livebearers and killifish
Advantages: Works well with digging fish, chemically inert
Disadvantages: Cannot be used with an undergravel filter, high reflectivity may wash out the colours on some fish
Best with: Any digging fish not requiring the pH/hardness buffering capability of coral sand
Advantages: Works well with digging fish, acidifies the water, many fish have particularly bright colours when kept over a dark substrate
Disadvantages: Colours the water, loose pieces of peat can clog the filter, reduces the pH rapidly so additional pH stabilisation may be necessary, cannot be used with undergravel filters
Best with: Killifish, Mormyrids, and some dwarf cichlids
Advantages: Supplements plain gravel to promote plant growth
Disadvantages: Relatively expensive, cannot be used with an undergravel filter
Substrate: Pond or aquatic soil
Advantages: Supplements other substrates such as gravel or sand to promote plant growth, inexpensive and easy to obtain
Disadvantages: Cannot be used with undergravel filters
Best with: Small, non-digging fish in a planted aquarium