Seiryu stone is the rugged grey rock, often shot through with white veins, that defines the classic Japanese aquascape and the Iwagumi layout in particular. It is limestone-based, so the one thing to know before buying it is that it will harden your water slightly.
What Seiryu stone looks like
Sharp, angular grey rock with deep crevices and pale veins running through it. Aquasabi calls it quite a compact stone, though still workable with hammer and chisel, where dragon stone crushes into fragments quite easily. Those jagged edges are why it reads as a mountain range in miniature, and why it became the default choice for the stone-led layouts Takashi Amano popularised. Buce Plant, which sells it, notes that the colour deepens when wet.
Seiryu, Ryuoh and dark Seiryu
The names cause more confusion than the rock. Most shops sell Seiryu and Ryuoh as one stone: Aquasabi lists “Ryuoh or Seiryu”, and Horizon Aquatics sells Seiryu “also known as ryuoh”. ADA, though, sells Ryuoh under its own name, and Terrarium Tribe reports that some in the trade believe the Seiryu sold outside Japan is really Ryuoh, a duller grey stone with fewer white veins. In the tank they behave alike: Aquasabi and ADA both say Ryuoh hardens the water. You may also see the names Seiryu-Seki and Mini Landscape Rock.
Dark Seiryu is sold as a darker form of the same stone. Horizon Aquatics says it is natural Seiryu put through an acid wash, which dulls the bright calcite veins to leave a charcoal or slate-grey stone with the same sharp structure. Not every source agrees on how it is made: Aquasabi and Buce Plant describe their dark versions by colour alone, and Terrarium Tribe says it couldn’t find what black Seiryu is or where it comes from. Either way, Horizon and Buce Plant say the dark stone still raises pH and hardness slightly, so choosing it for the colour won’t get you a softer-water rock.
It hardens your water
This is the part that catches people out. Seiryu contains a little lime, and Aquasabi says it may harden the water a bit. Horizon Aquatics says the same of both standard and dark Seiryu: either may slightly raise pH, KH, and GH. In a low-tech tank, that is a small effect.
There is also a ceiling. Aquarium Science says calcium carbonate, Seiryu included, dissolves only enough to bring water to about pH 7.6 to 7.9, no higher. It calls that a fine pH for adult fish, though a few blackwater species won’t spawn in it. Both hardness readings rise because of how the lime dissolves: Knott explains that it releases calcium, which raises GH, and carbonates, which raise KH. ADA adds that a lot of white on a stone doesn’t mean a bigger rise.
CO2 injection changes the picture. The carbonic acid it produces dissolves the lime in the stone, which raises carbonate and total hardness further. In Aquasabi’s article with aquascaper Oliver Knott, calcareous stones “tend to significantly harden the water if a CO2 system is used”, while the influence may be smaller without it. ADA agrees, and says the rise is particularly marked with Ryuoh. Horizon Aquatics takes the opposite view, that the effect is usually minimal in planted tanks with CO2 and regular water changes.
The standard counter is changing larger amounts of water frequently. Aquasabi recommends refilling with demineralised or reverse-osmosis water, and Knott adds that active soil helps pull minerals back out of the water. Aquasabi’s Seiryu listing puts the routine at about 50% of the water each week.
Preparing and placing it
Rinse it well under running water before it goes in. Aquasabi says this removes the fine dust that would otherwise cloud a new tank. There’s no need to boil or bleach it: Aquarium Science says rocks are fine as they are, even ones that fizz in acid.
Seiryu is compact and hard to break. Aquasabi says it can still be worked with hammer and chisel, while Terrarium Tribe finds it extremely hard to break and advises buying the sizes you need. Terrarium Tribe also warns that a falling piece can crack the glass, so it suits upright stones better than stacks or caves. Its advice is to anchor each stone by burying its base slightly and packing substrate around it.
For how much to buy, Aquasabi’s rule of thumb is your tank’s volume in litres divided by six, in kilograms. Seiryu needs no adjustment to that figure, where lighter lava would need less.
Using it in an Iwagumi
Seiryu is one of the stones Aquasabi names for an Iwagumi, a layout built from a single type of stone with no wood. Its guidance is to use an odd number of stones in different sizes: one large main stone (Oyaishi), a smaller secondary stone (Fukuishi) as its counterpoint, and small supporting stones (Soeishi and Suteishi). Let the lines in the stone’s texture guide where each one goes.
ADA’s advice for Ryuoh is to follow the ridge line and keep the stones looking continuous, and to have plenty of sizes with a similar texture to choose from. Horizon Aquatics notes that dark Seiryu is often picked for the main stone, with lighter Seiryu around it.
When to choose it, and when not to
- Good fit: hard-water fish, tanks where you are buffering hardness deliberately, and anyone happy to keep up frequent water changes.
- Think twice: soft-water species, shrimp tanks, and high-tech tanks running CO2, where stable, soft water matters more than the look of the rock.
- The alternative: dragon stone is inert and gives you a knobbly, cave-like texture instead of sharp peaks.
Whichever you pick, use one rock type throughout the scape. Aquasabi warns that mixing different types of stone costs a layout its harmonious effect. There is more on choosing rock in the aquascaping rocks guide.
Seiryu stone questions
Does Seiryu stone raise pH?
Yes, slightly. Horizon Aquatics says Seiryu may slightly raise pH, KH, and GH, and Aquarium Science says calcium carbonate stone can’t push pH above about 7.6 to 7.9. Aquasabi and ADA say the effect is stronger if you inject CO2. Frequent, larger water changes with demineralised or reverse-osmosis water keep it manageable.
Is Ryuoh stone the same as Seiryu stone?
Most shops treat them as one. Aquasabi and Horizon Aquatics sell Seiryu and Ryuoh as the same stone, though ADA sells Ryuoh under its own name and Terrarium Tribe reports that some in the trade think they differ. Both harden the water.
Is Seiryu stone safe for shrimp?
It can be, but it isn’t the easy choice. Horizon Aquatics advises checking how the stone has affected your water before adding shrimp, because a higher KH affects Caridina shrimp. An inert stone such as dragon stone or lava rock is the simpler option.
Sources: Aquasabi, Types of hardscape; Aquasabi, Choose your hardscape by weight; Aquasabi, Oliver Knott on water-hardening stones; Horizon Aquatics, Dark Seiryu stone vs Seiryu stone; Terrarium Tribe, Seiryu stone hardscape guide; Aquasabi, Seiryu Ryuoh (shop listing); Aquasabi, Hardscape; Aquasabi, Iwagumi; ADA, Hardscape materials (via Aquasabi); ADA, Layout materials: stones; Horizon Aquatics, Seiryu stone (shop listing); Aquarium Science, Rocks; Buce Plant, Boutique Seiryu Stone (shop listing); Buce Plant, Black Mountain Seiryu Stone (shop listing).