Sand for Shell-Dwellers: What to Use and How Deep

With shell-dwellers the substrate is not a design choice, it is the fish’s home. They dig in it, bury their shells in it, sift it through their gills and draw territory lines across it. Get the sand right and the colony settles fast. Get it wrong, too coarse, too shallow, the wrong colour, and they stay skittish and off-colour for weeks. I learned this by getting it wrong first, so here is what actually works in my hard-water shellie tanks, and why.

Why it has to be sand, not gravel

Shell-dwellers are diggers by trade. In the lake they excavate pits around their shells, bury the shell to the angle they like, and flick sand at each other to settle disputes. Gravel stops all of that. The grains are too big to shift, so the fish cannot dig; waste and leftover food drop into the gaps and rot where the fish cannot reach; and a shell sitting on gravel wobbles instead of bedding in. I have never seen a shellie look right over gravel. Sand lets them behave like themselves, and a sand bed with a healthy snail and micro-critter population also stays cleaner than people expect, because the fish keep the top layer turning over. If you want the wider picture on substrates across different tanks, the substrate guide covers the lot.

Grain size: fine, but not dust

Aim for a grain of roughly 0.5 to 2mm. That is fine enough to dig and sift, coarse enough to stay put and not cloud the water at every swish of a tail.

Two traps either side of that range. Very fine play sand, the sort sold for children’s sandpits, can be so powdery that a single startled dash kicks up a sandstorm that takes hours to settle and can clog a sponge filter. On the other side, “aquarium sand” that is really fine gravel is too big for small shellies to work properly. The sweet spot in my experience is pool filter sand, which runs around 0.4 to 0.8mm. It is graded, it digs beautifully, and it drops back out of the water almost at once if it gets stirred. That is the bag I reach for most.

The book behind Lagonia

The Hard Water Aquarium

My practical guide to working with hard, alkaline tap water instead of fighting it: reading your water, choosing and setting up the tank, and keeping and breeding the fish that thrive in it, from livebearers to Rift Lake cichlids and shell-dwellers.

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Colour matters more than people expect

This one surprised me. Bright white sand looks clean and crisp in the shop and it is a poor choice for the fish. Over a glaring white base, shellies wash out their colour and tend to stay nervous, because there is nowhere visually to hide and the light bounces straight back up at them. A natural buff, tan or pale gold tone is far kinder, and the fish show far better colour over it.

Very dark, almost black sand does the opposite and makes a multi or an ocellatus glow, but it can look stark and it shows every speck of detritus. I have settled on a mid, natural sandy colour as the honest compromise: the fish are relaxed and well coloured, and the tank does not look like a permanent cleaning job. Whatever you pick, think about the fish first and the photo second.

Will the sand change my water?

This is where hard-water keepers can relax a little, but it is worth knowing which way a sand pushes your chemistry.

• Quartz or silica sand (including most pool filter sand): chemically inert. It does not move your pH or hardness either way. On my hard, alkaline tap that is exactly what I want, because the water is already where the fish need it and I just want the sand to sit there and be sand.
• Aragonite or crushed coral sand: slowly dissolves and pushes KH and pH upward over time. If your water is only moderately hard, or your pH sits a touch low like mine, this is a genuinely useful hands-off way to hold the water up in the Tanganyikan range. I sometimes mix a little into an inert sand bed for exactly that reason. The method sits alongside the rest of buffering hard water for Rift Lake tanks.
• Planted-tank “aqua soils” and some black “aquarium sands”: these are often designed to lower pH for soft-water plant tanks. That is the wrong direction for shellies, so read the bag and avoid anything sold as buffering pH down.

If you are unsure what your tap is doing in the first place, the GH and KH explainer sorts out which number is which, and the remineralise calculator helps if you are building hardness up from soft water rather than working with hard tap.

How deep to make it

Depth is the mistake I see most, and the one I made myself. Go at least 5cm, and more if your tank can take it. Shallow sand looks tidy and fails the fish: they cannot dig a proper pit, they cannot bury a shell to the angle they want, and a half-buried shell keeps toppling. My first shellie tank had barely 2cm and the colony never truly settled until I stripped it and started again deeper.

I now bank the sand a little, shallower at the front glass where I want to see the fish, deeper at the back and in the corners where the shell clusters go. That gives them a proper 6 to 7cm to work in around the shells without burying the whole tank. To work out how many kilograms a given depth needs before you buy, the substrate calculator saves you a wasted trip or an extra bag.

Rinsing and keeping it clean

Rinse new sand until the water runs clear before it goes anywhere near the tank. This is tedious and non-negotiable; unrinsed sand, pool filter sand especially, will cloud a tank for days. I do it a few kilos at a time in a bucket, swirling and tipping off the milky water over and over until it comes out clean. Patience here saves you a fortnight of murk later.

In an established tank, do not deep gravel-vac sand the way you would gravel. Driving a siphon down into the bed just pulls your sand out and disturbs everything. Instead I hover the siphon a centimetre or two above the surface during a water change and let it lift the light detritus sitting on top, and I give the bed a gentle stir with a finger now and then to release any trapped gas. The fish do most of the real turning over themselves. That light-touch routine fits straight into a normal water change and needs no extra work.

What I actually use, and what it costs

No mystique here. For most of my shellie tanks I buy plain pool filter sand or washed quartz building sand, both of which cost very little per bag compared with branded aquarium substrate, rinse it hard, and lay it 5 to 7cm deep. Where I want a gentle push on pH I blend in a smaller quantity of aragonite or crushed coral. That is the whole recipe. The fish do not read the label, they read the grain size, the depth and the colour, and a cheap bag that gets those three right beats an expensive one that does not.

Once the sand is down and the water is right, the rest of the build is the easy part. If you are starting a colony from scratch, the full shell-dweller tank setup walks through tank size, filtration and stocking, and the best shells for shell-dwellers covers the other thing they cannot live without.