Key Takeaways
- People use gravel in pond construction to hide the black liner and shade it from sunlight. A thin rounded layer suits a wildlife pond or a lightly stocked garden pond where the bottom is visible.
- Gravel on a pond floor needs the stones lifted and washed every two to three years because it traps leaves, fish waste and silt in the gaps between the stones.
- A koi pond or any heavily stocked fish pond is better off with a bare liner and a proper filter, because waste trapped in gravel feeds algae and gives parasites somewhere to sit.
- Limestone, marble and any stone that fizzes under a drop of vinegar belongs outside a pond, because the stone dissolves slowly and pushes the water alkaline. Granite, quartzite, flint, basalt and slate stay inert in water.
- Rounded, washed gravel of 10 to 20mm is the safe size on a pond liner, with larger pebbles kept for the shelves and edge, since crushed angular chippings have edges that can puncture the rubber.
- On a pond floor, lay 10 to 20mm in a wildlife pond; in a fish pond, nothing or one layer of large smooth cobbles.
Many years ago, long before I started Outdoor Aggregates, I was called back to a pond I’d installed for a client because it smelled and they wanted it looked at. The pond itself was doing fine, but when I started lifting the gravel, I found exactly what I’d seen before: pea gravel mixed with years of fallen leaves and debris had turned into a thick, smelly goop. The owner had put 15cm of gravel in soon after the pond was filled, mainly to hide the black liner showing. It had looked lovely enough at first, but there hadn’t been much thought about what would happen down the track. I spent most of the day washing the stone so it could be reused and only added a thin layer of 10 to 20mm back. Since then, I’ve been more careful about where gravel goes in a pond and, just as importantly, what sits above it. Most of the stone belongs on the shelves and edges, not the deep floor. This guide is for people looking to use gravel in their pond.
Should You Put Gravel in a Pond?
Gravel in a pond is worth it for a wildlife pond or a lightly stocked garden pond, but a mistake in a koi pond. The difference is what falls into it. Two goldfish produce waste at about the rate the gravel and plants break it down. Twenty koi produce it faster than anything can cope with, and gravel gives it somewhere to hide.
Three questions decide it for me:
- Can you see the bottom when the water is clear? If yes, gravel earns its place on looks alone. If the pond is big and murky, nobody will see it and it only adds places for waste to collect.
- How many fish? Two or three goldfish in a planted pond is fine with gravel. A koi pond, or anything you feed every day, wants a bare liner and a filter doing the cleaning.
- Will you lift and wash it? Every two to three years the gravel comes out, gets washed and goes back. If that sounds like a nightmare of a job, leave the liner bare and plant it out heavily instead.
If you said yes two out of three, I would put gravel in. The people who regret it answered the third question wrong.
Gravel or Bare Liner: Which Suits Your Pond?
A bare liner suits a pond you clean like a big aquarium, and gravel suits a pond you let settle into itself. The trade is looks against cleaning, and this is how I laid it out for clients who could not decide.
| Pond floor | Looks | Cleaning | Liner protection | Suits |
|---|---|---|---|---|
| Bare liner | Black, artificial until algae films it | Net and vacuum, no lifting | None from sunlight | Koi ponds, formal ponds, minimal upkeep |
| Thin pea gravel, 10 to 20mm | Natural, hides the liner | Lift and wash every 2 to 3 years | Shades the liner, holds it down | Wildlife ponds, planted ponds with a few goldfish |
| Pebbles and cobbles, 20 to 40mm | Natural, easier to see between | Rinse in place, lift less often | Shades and weighs down the liner | Beaches, shelves, ponds where fish root about |
| Larger rocks with gravel between | Most natural, hides pump and pipe | Hardest to clean, gaps trap debris | Full cover, heavy rocks need a pad | Big informal ponds fed by a stream or waterfall |
The second row is where I would steer most people. A thin layer of pea gravel gives the natural look, and because it is thin it moves aside with a hand or a soft brush when you need to get at the liner. The gravel that is easy to live with is the gravel you can push around.
Does Gravel Trap Muck in a Pond?
Gravel on a pond floor does trap muck, because leaves, fish waste and silt drop into the gaps between the stones and stay there. Anyone who tells you gravel keeps a pond clean is describing a pond where a filter does the real work.
The argument in favour is about bacteria. Every wet surface in a pond grows the bacteria that break down fish waste, and a floor of gravel has far more surface than a flat sheet of rubber. That is true. Where I part company with the gravel-sells-itself crowd is on how much it matters. In a small planted pond the gravel keeps up with two goldfish. In a stocked fish pond the bacteria on the gravel are a rounding error next to a proper filter, and the muck the gravel holds does more harm than they do good. Gravel is a home for bacteria. It is also a home for everything else.
Which Stone Is Safe in a Pond?
The safe stones in a pond are granite, quartzite, flint, slate and basalt, because none of them dissolves in water, altering the pH. Limestone and marble do, and they are the two stones people most often tip into a pond by accident because they came out of a rockery. Limestone is calcium carbonate. Sat in water it dissolves slowly and the water goes hard and alkaline. Fish tolerate a surprising range, but the plants tend to suffer and you lose any say over where the chemistry ends up. I always steered clients off it near water.
The test I use is a splash of white vinegar on a dry stone. If it fizzes, the stone is limestone or marble and it stays out. The fizz is the stone dissolving.
| Stone | Safe in a pond | Why |
|---|---|---|
| Quartzite pea gravel | Yes | Inert, rounded, washed |
| Granite | Yes | Inert and hard, nothing leaches |
| Flint and quartz pebbles | Yes | Inert, naturally rounded |
| Basalt | Yes | Inert, dense |
| Slate | Edge yes, floor not without a lot of care | Inert, but sharp edges risk the liner and trap silt |
| Limestone and Cotswold chippings | No | Dissolves, raises hardness and pH |
| Marble and white spar | No | Calcium carbonate, same problem as limestone |
| Sandstone | Check | Soft grades crumble and cloud the water |
Slate needs its own line. I love it as coping (the flat stone capping the rim of the pond) and in a stacked waterfall, but split slate has an edge like a blade and a loose layer of it on the floor is asking for a puncture. I keep slate at the margins, or very carefully select the stones to avoid any sharp edges making it on to the floor. The full case, including the pyrite check, is in the slate water features guide.
Pea Gravel, Pebbles or Rocks: What Size for a Pond?
Rounded 10 to 20mm gravel is the size I would put on a pond floor, with pebbles of 40mm and up where fish root about or a shelf needs weighing down. Shape matters more than the number. Rounded stone has been tumbled by a river or a quarry and has no edge to speak of, while crushed chippings are broken rock and every piece has a corner.
Smaller gravel shifts about, gets carried off by fish and disappears up a pond vacuum, so 20mm is the better pick if you vacuum the floor. Laid thick, small gravel also stops water moving through it, which is why 10 to 20mm is plenty. Also, anything that needs two hands to lift stays off the liner and sits on a concrete pad with old carpet under it.
Gravel in a Wildlife Pond
Gravel in a wildlife pond earns its place on the shallow beach more than on the floor, because the beach is where frogs, toads and birds get in and out. One side of the pond slopes gently up to the edge, dressed with rounded pebbles and cobbles and a few flat riven slabs, so anything that falls in can walk out. It’s probably best to skip slate there: it is slippery wet and hot in sun, and rounded stone gives soft feet something to grip.
On the floor, a wildlife pond is the best case for gravel. No fish to feed, plants doing the filtering, and the muck that gathers in the gravel is exactly the stuff pond life lives in. Put 7 to 10cm of loamy soil in the deepest part for the deep-water plants, gravel over the rest, and let it go. The water clouds for a week or two and then clears.
How to Lay Gravel in a Pond
Laying gravel in a pond is a bucket-and-hands job, and the two things that matter are washing the stone first and never tipping a bag straight onto the liner. This is the order I worked in:
- Wash the gravel: Half fill a bucket, top up with water, stir hard, pour off and repeat until it runs clear. Quarry dust turns a new pond to milk for a fortnight.
- Check what is under the liner: A new pond wants 5cm (2 inches) of damp sand or a proper underlay beneath the rubber. On an existing pond, smooth out creases and folds before the gravel goes on.
- Carry it in by bucket: Lower each bucket to the floor, tip gently and spread by hand. A bag emptied from the edge drops 20kg on one spot.
- Keep it thin: Spread to 10 to 20mm in a wildlife pond, just enough to hide the liner; in a fish pond, nothing or one layer of large smooth cobbles.
- Do the shelves and the edge separately: Larger pebbles weigh down the planting shelves, and the coping stones at the rim overhang the water by 2.5 to 5cm so the liner stays hidden when the level drops.
- Fill slowly and wait: Run the hose onto a slab or cover the area that the water runs in so it does not scour the gravel, expect cloudy water for a few days. Always give tap water two weeks before any fish go in.
A 20kg bag of 10mm gravel covers about two thirds of a square metre at 20mm deep, so a 2m by 1.5m floor at 10 to 20mm takes about three to five bags. Our gravel calculator does the sums easily for you.
How to Clean Gravel in a Pond
Cleaning gravel in a pond means stirring the sludge up and pumping the dirty water out, and a pressure washer is the one tool to keep away from the liner. On a full pond, a pond vacuum over the gravel every few weeks in autumn pulls out the leaf litter before it rots. On a pond that has gone too far, the fish go into a holding tank, the pond drains, and the gravel gets hosed and stirred with the old pond water until the muck lifts and can be pumped away. The film of bacteria on the stones is the part that was working, so shift the waste and leave the stones alone, and skip detergent and bleach because whatever goes on the stone ends up in the water.
Every two to three years the gravel comes out for a proper wash: shovel it onto a tarpaulin, dry sieve it so the fine sludge falls out, then bucket or power wash it until the water runs clear. Outdoor Aggregates has the various ways to deep clean in how to clean garden stones and gravel. If the gravel has turned to porridge all the way through, price up a fresh bag before you start sieving. A bag of clean 10mm costs less than a lost weekend.
Next Steps
Rounded quartzite is the stone I would use on a pond floor, and Outdoor Aggregates sells it as 10mm gravel, a washed, pH-neutral pea shingle in 20kg mini bags or an 850kg bulk bag from £127 including VAT. The 20mm gravel is the better pick for a floor you vacuum, at £142 a bulk bag. If you are still deciding between a pond, a stream and a fountain, start with the garden water features overview. We would rather tell you to leave the liner bare than sell you a bag you end up sieving.
If you would rather talk the gravel for your pond through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.
Have you ever put gravel in a pond? What size did you use, and how has it held up after a couple of winters? Let us know in the comments.
FAQs
Do fish prefer gravel or sand in a pond?
Pond fish do better over gravel than sand, because sand compacts into a solid sheet that holds waste on top and clouds the water every time a fish stirs it. Goldfish root through 10 to 20mm gravel for food, which sand does not allow.
Should I put stones in the bottom of my pond?
Stones in the bottom of a pond are fine if they are rounded and sit on a protected liner, and a poor idea if they are large or angular. A rock big enough to need two hands belongs on a shelf or at the edge, on a pad of old carpet, where it holds the liner down without pressing a point into it.
Does gravel help with algae in a pond?
Gravel in a pond does nothing for algae on its own. Green water is sun and nutrients outpacing the plants, so the fix is more plant cover, some shade over the surface and a pump moving the water.
Can you put beach pebbles in a pond?
Beach pebbles can go in a pond once they are washed until the water runs clear and checked with a drop of vinegar, since chalk and limestone pebbles fizz and stay out while flint and quartz are fine. I would buy them bagged: some British beaches ban taking pebbles, plus, the stones are part of the coast’s defence.