Frequently asked questions on aquarium substrates


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Gravel, sand, specialist planting substrates - there's a lot of choice out there when it comes to what to put at the bottom of your tank. Matt Clarke answers some frequently asked questions about aquarium substrates...

How should I clean my substrate before I add it to my tank?

Although they're often pre-washed, most substrates are very dusty and need to be cleaned thoroughly before they're used, otherwise the tank will turn extremely cloudy.

Washing dusty gravel or sand is messy, tedious and, in winter, rather cold work. The best way to get the substrate clean is to place a small quantity (say a few mugfuls) in a clean bucket and spray water on to it using a hosepipe.

You'll need to keep swirling the gravel around with your hand, and pouring away the dirty water until it runs clear. Then you can tip the washed gravel into another bucket and clean the next batch.

Some substrates don't respond well to being washed in this way, so the more you swirl them, the dirtier the water gets.

Powder-coated coloured gravels are a particular irritation here, so you may as well just get rid of as much of the dust as possible and then just give up, or only swirl very gently.

Laterite substrates are impossible to wash...

How much gravel do I need to buy?

This is harder to work out than you might think, because although sold by weight, it's the depth (or volume) of the substrate that really matters to us fishkeepers, not the weight.

For sand, most people go for a depth of around 2.5cm/1", but with gravel the norm is to go for a deeper layer of say 5cm/2" or more.

The weight of a litre of dry substrate varies from about 1.95kg per litre for fine sand to just 1kg per litre for baked clay substrates. To work out how much you need, simply determine the volume you require and multiply it by the weight of a litre of the substrate of choice.

I've added coloured gravel to my tank and the pH has risen. Why?

Many coloured gravels, particularly the rough textured angular ones with a powdery colour coating, are made from white dolomite. This is a naturally occurring mineral rich in calcium and magnesium, and in its uncoloured form is sold as a substrate for use in marine or Rift Lake cichlid tanks to keep the pH, alkalinity or hardness high.

If you already live in a hard water area, or keep fish that don't mind alkaline conditions such as goldfish, it's not likely to cause any problems. However, it wouldn't be a great choice at all if you want to keep fish that like neutral or soft water - you'll need an inert substrate instead.

How can I keep my substrate clean?

The easiest way to keep the gravel clean is to regularly use a gravel cleaner. In my experience, the battery- or air-powered varieties are invariably much less effective than the cheap, heavy duty syphon-powered ones. The heavy duty Hagen gravel cleaner I bought nearly a decade ago is still going strong, so these are remarkably good value gadgets. I use mine to vacuum my substrate once each week and my tanks always looks clean.

How do I use a gravel cleaner?

Syphon-powered gravel cleaners are designed to remove dirt-laden water from within the substrate when you're doing your partial water change each week. The power of the syphoning water causes the gravel grains to swirl around inside and sucks the dirty water out, but leaves the substrate behind.

Once you get the hang of it, you should be able to vacuum most of the tank floor while only removing about 20-30% of the aquarium water, which you can then top-up with dechlorinated tapwater.

You can start a syphon-powered gravel cleaner in several ways. Since I have inadvertently inhaled several lungfulls of foul tasting aquarium water in the past, I personally prefer not to suck on the end of the hose to get the syphon going - although this is undoubtedly the quickest and most widely used method among fishkeepers.

To start the syphon more hygienically, put your thumb over the end of the hose and submerge the gravel cleaning attachment in the tank on its side. If you quickly scoop up some water inside the attachment and allow it to fill the hose by temporarily removing your thumb a couple of times, you should end up with both the attachment and the hose completely filled with water. Then all you need to do is simply lower the hose end over a bucket and release your thumb. The water should start flowing on its own.

Some substrates are very coarse. What diameter should I go for?

The gaps, or interstices, between gravel grains increase as the grains get bigger, so more dirt may get trapped in the substrate if you use a larger grain size. In contrast, very fine substrates, such as sand, don't allow much dirt to be trapped, so detritus tends to sit on top, where it can be sucked into the filter or syphoned off.

Some fish, including many common species like goldfish, feed by sifting the substrate and eating any food items found within. Many catfishes, like Corydoras, and some oddballs such as rays, eels and mormyrids such as elephant noses, also like to root about in sand.

How can I replace my substrate with something more attractive?

The easiest way to remove the old substrate, without the need to strip down the tank, is to simply syphon it out. If you find a suitable pipe (you'll really need one thicker than the average gravel cleaner hose), you should be able suck up the substrate, the dirt and some of your water.

You can then carefully add your washed substrate to the tank and top up with dechlorinated tapwater that matches the temperature of your tank. The snag with this method is that you sometimes need to remove a lot of water in order to suck all of the substrate out.

You might need to top up and wait a few days before removing the rest to avoid stressing the fish by changing too much water in one go.

Alternatively, you could try removing the remainder with a net.

That white coral sand could give my tropical tank a really marine look. Is it safe to use?

Not unless you want to make your water rock hard and increase the pH of your water to over 8.0. Coral sand and gravel is calcareous, so you should only use it in situations where you want the pH and hardness to be kept high. Many Rift Lake cichlid hobbyists use it to provide the appropriate conditions for their fish.

Those in very soft water areas, such as parts of Scotland, also use small amounts to stop the pH of their aquaria from dropping naturally, something which occurs due to the lack of buffering in their water.

How deep should my silver sand be?

In freshwater tanks, generally only a thin layer of sand is used, since it is so fine that it tends to stagnate if used in a deep layer. About 2.5cm/1" should be plenty in most cases.

I've added an expensive plant substrate. Can I still use a gravel cleaner?

If you use a gravel cleaner in a tank containing a costly planting substrate, such as laterite, you risk sucking it out. If you've got a professional-style planted tank, chances are you'll be dosing with CO2 too, and you'll have a lower stocking density, so this shouldn't be too much of a problem.

What is an inert substrate?

Inert substrates and rocks don't alter the water chemistry, while non-inert, calcareous substrates do. These contain calcium and other minerals that increase the hardness and pH, so if you add them to your tank they'll make the water harder and more alkaline.

Inert substrates are of particular importance to those keeping fishes that like soft water, such as Discus. Even standard pea gravel contains sufficient calcium to boost the pH and hardness, so don't use it unless you want hard water.

Can I use a mixture of grain sizes?

You can, but they'll all get mixed up eventually with the larger grains usually rising to the top. Personally, I think a mixture of different grain sizes looks very natural, especially in a reef tank.

How do I clean sand?

Before it's added, sand needs to be cleaned thoroughly by swirling it around in a bucket of water until the water runs clear. Once it's in the tank it's a doddle to keep clean. I tend to give it a gentle stir with my index finger while holding a syphon hose in the same hand, which allows any trapped particles to be sucked out.

It's impossible to avoid removing some of the sand during maintenance, as it's light and very easy to suck up, so you'll need to top it up occasionally.