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Sand As Substrate: A Quick Primer

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A stroke of the brush does not guarantee art from
Jul 16, 2005
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Berkhamsted, Hertfordshire, England
I wrote a piece for the January edition of Practical Fishkeeping, and seeing as questions about sand get posted here with some frequency, I thought I'd precis a little of that article here.

Why use sand?

Two reasons: it can look very nice, and it is better for fishes that dig or forage in the substrate. Sand, and especially sand with a few patches of gravel and pebbles, can look much more realistic than plain washed gravel. The particle size works well with smaller fish.

Catfish, gobies, soles, spiny eels, and cichlids are all examples of fish that manipulate the substrate. With sand, these fish can dig or 'earth-eat' without any risk or damage. Gravel wears down the barbels of catfish, and with spiny eels especially, skin wounds become a site for bacterial infections. In fact, I'd go far to say that sand is mandatory for spiny eels and 'freshwater' soles/flatfish.

What about plants?

Plants root very well in sand. I mix gravel and laterite or pond soil to make up a bottom layer, and then cover it with the desired depth of sand.

What about anaerobic pockets?

If organic material gets buried below the top 2 cm of sand, it can potentially turn nasty. Anaerobic decay is where bacteria consume the organic materia without using oxygen but producing gases such as hydrogen sulphide. Such gases can kill fish.

However, you can reduce the risk to zero by doing any one of a number of simple things:
  1. Use a thin layer of sand. Obviously if the layer of sand is only 1 cm deep, then anaerobic conditions cannot develop.
  2. Add burrowing fish. Things like Synodontis, Awaous gobies, kuhli loaches, and eartheater cichlids will turnover the sand constantly and remove any overlooked food. By sifting the sand, they also prevent anaerobic pockets from developing, so things like plant leaves and fish faeces decay aerobically.
  3. Use Malayan livebearing snails. They will constantly turn over the sand, letting oxygen get in, and will also eat up any bits of waste that gets into the sand. These snails are a true blessing in the sandy aquarium!
  4. Use live plants. The roots of plants have evolved to live in dysaerobic or anaerobic substrates, and carry oyxgen from the water into the substrate. This keeps the substrate sweet. You won't get anaerobic decay in a well-planted aquarium.
  5. Use an undergravel heater. Warm water currents circulate in the gravel (convection currents) and these draw oxygen into the sand. Again, this prevents anaerobic conditions from developing.
What about silt?

Sand contains some silt and will cloudy the water. This seems not to do the fish any harm at all. Use filter aids if you want (these clump silt particles together so that the filter removes them more quickly). Certainly change the filter wool a couple of times, as filter wool is the best thing for removing silt quickly.

What kind of sand?

In a hard water tank, use coral, reptile, river, or silica sand as you prefer. In a soft water aquarium, you need a sand lacking in lime (calcaereous minerals), so stick with silica sand. Silica sand is sometimes called silver sand. There is an idea going about that silica sand promotes diatoms, but I have never observed this to be the case, and I think the 'science' behind it is very fuzzy. Assuming the silica sand you obtain is pure quartz, it is no more soluble than the glass (also a silicate) that makes up your fish tank.

Any downsides?

Sand tends to be more reflective than gravel, and many fish object to this, and will show faded colours. It cannot be used with an undergravel filter (coral sand excepting). Some plants do not like having sand between their leaves (specifically, Vallisneria). It doesn't come in a variety of hideous colours like turquoise blue or psycho pink.

My aquarium, with sand, shored up with boulders and some patches of gravel.

One option to avoid anaerobic pockets is to do exactly this, and have a coarse substrate below and a finer one (not silica sand, but certainly coral sand or fine pea gravel) on top, with the two layers separated with a "gravel tidy", basically a piece of plastic mesh. This can work very well.

In my tank, the rockwork (lava rock) and coarse gravel certainly did have air pockets in for the first few days, at least betweent he sand and the glass. But they worked themselves out, and there aren't any anymore. Whether the air simply rises, or the snails disturb it, I don't know.

Here are a couple of pictures that might be useful. The first shows a "section" through the substrate at the deep end. The black stuff is aquatic soil (pond soil). The lighter coloured chips of gravel are obvious. Then the sand goes on top. It seems pretty stable, and if you look close at the real thing, you can see snail burrows and plant roots. The second shot is of the surface, with a little gravel scattered on the sand to blend the sand in with the boulders. I haven't done this all over the tank, just in certain spots, to create an effect a bit like you see in nature, where there are exposed patches of gravel peeking out from the sand or mud.



A very good guide, but can you put 1cm of sand ontop of whatever you put under the sand without worrying about them air pockets? Like on your tank Neale, is it ok to do that if you just put 1cm of sand ontop of whatever you use?
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