The materials list · Hardscape

Aquarium rocks and wood: compare materials

The finder is for deciding. This is for reading. Nothing is ever removed from this page, including the four materials that must never enter a tank, because you cannot judge a catalog you cannot see. Open any row for the preparation, the naming trap, the Armenian note and the sources behind it.

Sort by the confidence column first. It is the column the rest of the trade does not print. Sorting it descending puts the low confidence rows at the top, and every one of them is a stone whose composition nobody has published: a trade name, a seller's description, and no analysis. Those rows are not marked inert and they are not marked dangerous. They are marked unknown, which is the only honest thing to call them, and the finder holds them out of any search for inert stone rather than letting them pass.

Every field

Click any heading to sort. Click again to reverse it, once more to return to the order the file was written in.

File order is deliberate: the international trade names first, then the Armenian stone, then the wood, then the four that must never go in a tank. Materials with no recorded value for the column you sorted on sink to the bottom in both directions. The GH and KH column sorts inert, slightly, strongly, unknown, which puts the untested rows at the end where they will actually be read.
Iwagumi rock detail
Photo: Moritz Holzinger · CC BY-SA 3.0

Read the confidence column as part of the chemistry, not as a footnote to it. The two columns only mean something together. Inert at high confidence is a fact. Inert at low confidence is a seller's opinion. Unknown at low confidence is the state of the published record for four of the trade's better known stones, and it is the reason the fourteen day soak protocol exists.

How to read it

Five columns carry more weight than the rest.

GH and KH

Four values, not three. Inert, raises slightly, raises strongly, and unknown. Unknown means no published composition, and on this site it is never rendered as inert, never rounded down, and never treated as safe. Both of the things a carbonate rock does come out of one reaction, so GH and KH move together at close to one to one in German degrees1.

Confidence

Carried straight through from the research file's own rating, which grades chemistry and geology separately where they differ22. High means the behavior follows from an identification nobody disputes. Medium means the direction is known and the proportion is not. Low means a trade description and nothing behind it.

Ceiling applies

Yes wherever the material contains, or may contain, calcium carbonate. Where it is yes, the material cannot push your water past the calcite ceiling: about 2.7 dGH uninjected and about 9.9 dGH at 30 mg/L of CO221. Surface area changes how long that takes and changes nothing about where it ends.

Smallest scale

The smallest scape the material is normally usable in, at the piece sizes it is sold or found in. A larger tank accepts everything at or below it. A large piece in a small tank is the mistake; a small piece in a large tank is only a composition problem, so the finder never removes those.

In Armenia

Collectable means you can pick it up in this country. Sold means it appeared in the only specialist catalog in the country, which holds 16 stone products and one driftwood listing20. Import only means neither, and it is the most common value in the wood half of the table.

The pattern in the chemistry column

Sort by GH and KH, then read the In Armenia column beside it.

The inert block and the collectable block overlap almost exactly. Basalt, volcanic scoria, obsidian and pumice are all inert, all Armenian and all free. The materials people pay to import are, with one or two exceptions, the same rocks with better names on them.

The pattern in the price column

Sort by price band ascending.

Everything in the free band is Armenian. The wholesale price of branded aquascaping stone at the Chinese quarry gate is 350 to 600 US dollars a tonne10, and European retail is 8 to 15 euros a kilogram. What you are paying for is freight, sorting, breakage and margin, none of which apply to a rock you carried home.

The pattern in the wood rows

Sinks and tannin move together, and both are mechanical.

The woods that sink on day one are the dense and the bog preserved, and they are also the ones that stain hardest. The pale branching woods float and stay clear. There is no wood that does both, which is why professionals screw the light wood to a slate base rather than waiting for it.

The four rows marked never

Three of them are Armenian and free.

Metallurgical slag, sulphide mineralised rock and evaporite are all lying around in this country, and untreated concrete is in every builders yard. They are in the table rather than left out, because the reason they are dangerous is that they look or sound like something good.

The gap in the record

Nobody has published a thin section of an aquarium rock.

That sentence is the reason this table has a confidence column at all. Origin and appearance for a trade name are trade descriptions unless a source is given. The water chemistry column is more reliable than either, because anyone with a test kit and a bucket can check it, which is exactly what the fourteen day soak protocol asks you to do.

Four rows are marked unknown at low confidence. Frodo is sold as metamorphic, its composition is not published, and reports of its acid behavior disagree22; it also returns zero results on the Chinese directory that carries hundreds of Seiryu and Ohko listings10, which suggests the name was invented at the retail end rather than the quarry. Yamaya, Manten and Koke are ADA trade names5 whose published record is the manufacturer's own visual description. Manten is reported inert by the people selling it. Koke's pale marking is consistent with carbonate, which is a hint and not a measurement.

Two more rows are unknown for a different reason. Armenian river stone is unknown per piece, because the Hrazdan catchment is fractured Quaternary lava, and carbonate rocks also outcrop in the Argichi, Vedi and Hrazdan basins19, so the geology is published and the pebble is not. Metallurgical slag is unknown because smelter waste carries whatever the smelter was processing13.

None of those six is marked inert anywhere on this site, and the finder holds all of them out of a search for inert stone unless you tick unknown on purpose. That is the opposite of how the fish finder treats a missing figure, and the reversal is deliberate: a missing fish costs you a fish, and an unpublished stone costs you the tank.

Fourteen days, RO water, no cost22

Day 0
rock in a bucket of RO or distilled water, nothing else in it. Record GH, KH and TDS. Cover it
Day 7
measure again
Day 14
measure again
Not tap water. Tap water may already be at or near saturation, and a rock cannot dissolve into water that has no room for it, so tap water will hide the very effect you are testing for. If nothing has moved in fourteen days the rock is inert in your conditions. Full protocol, and the limits of the vinegar test it replaces, on the finder page.

Where each column came from

Every row carries its own source ids, and every one of them is printed below.

Chemistry and the ceiling. Calcite behavior, the hardness conversions and gypsum as calcium sulfate come from USGS Water-Supply Paper 22541, with the equilibrium constants from a manufacturer's chemistry paper2. The soft to very hard classification3 is the USGS one. The acid test protocol and the cold dolomite limitation come from a geology reference4. The ceiling figures themselves are ours21.

Trade names. ADA's own descriptions of Ryuoh, Koke, Yamaya and Manten5 are the entire published record for three rows. The Saprolegnia warning on new wood is also the manufacturer's6. Rock type and plant genus definitions are Wikipedia entries used only where nothing is contested7. Where the rock is actually quarried, and at what price, comes from the Chinese export directories10.

Wood. The under one day tannin half life8 and the Rio Negro dissolved organic carbon result at pH 4.09 are both peer reviewed, and between them they replace most of what the hobby says about blackwater.

Armenia. The tuff and travertine naming trap, including the interwiki error itself, is Armenian Wikipedia11. Basalt and its 125 billion cubic meter reserve12, scoria and the 500 cones13 and the slag warning, obsidian14, the three carbonate rocks and the pyrite in Armenian marble15, the Avan salt sequence at 96.84 percent NaCl16 and the Kajaran sulphide ore17 are all from the same encyclopedia, in Armenian, and are treated as the object of study rather than as an authority. The contamination numbers are peer reviewed18, the tap water is a national academy paper19, and the Armenian prices and stock counts are one shop's own catalog20. The compilation, the confidence ratings and the row by row verdicts are this hub's research files22.

  1. Hem, J.D. (1985). Study and Interpretation of the Chemical Characteristics of Natural Water, 3rd edition. US Geological Survey Water-Supply Paper 2254. Public domain. Calcite solubility in contact with air, about 20 mg/L calcium from experiment and 16 mg/L from thermodynamic calculation, page 92; the hardness unit conversions; and gypsum as calcium sulfate. The paper our ceiling calculation is validated against. Full text PDF
  2. Morin, G. Understanding the General Chemistry of the Planted Aquarium. Seachem Laboratories. The equilibrium constants used in the ceiling calculation. PDF
  3. Hardness of Water. USGS Water Science School. The soft, moderately hard, hard and very hard classification. usgs.gov
  4. The Acid Test for Carbonate Minerals and Carbonate Rocks. Geology.com. The dilute hydrochloric acid protocol at 5 to 10 percent, the 5 percent acetic acid comparison, and the cold dolomite limitation. geology.com/minerals/acid-test.shtml
  5. Layout Materials: Stones. Aqua Design Amano. ADA's own descriptions of Ryuoh, Koke, Yamaya and Manten. adana.co.jp
  6. Layout Materials: Driftwood. Aqua Design Amano. The manufacturer's Saprolegnia warning on branch wood. adana.co.jp
  7. Rock, mineral and plant definitions. Wikipedia entries for Limestone, Dolomite, Mudstone, Basalt, Petrified wood, Slate, Quartzite, Bog-wood, Rhizophora, Arctostaphylos, Cylindropuntia and Grayanotoxin. Uncontested definitions only. Limestone
  8. Maie, N., Pisani, O. and Jaffe, R. (2008). Mangrove tannins in aquatic ecosystems. Limnology and Oceanography 53(1): 160 to 171. The under one day half life. DOI 10.4319/lo.2008.53.1.0160
  9. Duarte, R.M., Smith, D.S., Val, A.L. and Wood, C.M. (2016). Dissolved organic carbon from the upper Rio Negro protects zebrafish against ionoregulatory disturbances caused by low pH exposure. Scientific Reports 6: 20377. Open access. DOI 10.1038/srep20377
  10. Made-in-China business to business directory, product searches. Seiryu Stone, Dragon Stone, Ohko Stone, Frodo Stone and Aquarium Driftwood, accessed 8 September 2026. The single Hebei supplier, the 350 to 600 US dollars per tonne price with a one tonne minimum, and the zero result for Frodo Stone. Seiryu search
  11. Տուֆ and Հրաբխային տուֆ. Armenian Wikipedia, through the language links on both, accessed 8 September 2026. The composition of Armenian tuffs, the named quarry varieties, the statement that the group also covers calcareous travertine, and the interwiki mislabelling itself. hy.wikipedia.org
  12. Բազալտ. Armenian Wikipedia, accessed 8 September 2026. Basalt mineralogy, stability against acids and alkalis, distribution, and the 125 billion cubic metre reserve figure. hy.wikipedia.org
  13. Հրաբխային խարամ and Խարամ. Armenian Wikipedia, accessed 8 September 2026. The 500 scoria cones and roughly 300 million tonnes of reserves, and separately that bare խարամ is metallurgical slag. hy.wikipedia.org
  14. Վանակատ, with Frahm, E. and colleagues (2024). Armenian Wikipedia for the description and deposits; Scientific Reports 14: 9528 for the source complexes. Hardness 5, density 2500 to 2600 kg per cubic metre. nature.com
  15. Կրաքար, Տրավերտին and Մարմար. Armenian Wikipedia, accessed 8 September 2026. The three Armenian carbonate rocks, where each occurs, and the pyrite among Armenian marble's accessory minerals. hy.wikipedia.org
  16. Ավանի աղահանք. Armenian Wikipedia, accessed 8 September 2026. A 700 metre Miocene salt bearing sequence, worked since 1967, 96.84 percent NaCl, with Pleistocene dolomitic basalts in the same sequence. hy.wikipedia.org
  17. Քաջարանի պղինձ-մոլիբդենային հանքավայր. Armenian Wikipedia, accessed 8 September 2026. The Kajaran copper and molybdenum deposit in the upper Voghji basin, its ore minerals, and the related sulphide belts. hy.wikipedia.org
  18. Gevorgyan, G.A. and colleagues (2016). Environmental Risk Assessment of Heavy Metal Pollution in Armenian River Ecosystems. Polish Journal of Environmental Studies 25(6): 2387 to 2399. Every copper, zinc, cadmium, arsenic and nickel figure used here. pjoes.com PDF
  19. Shahinyan, H.V. (2005). On the hydrogeochemical characteristics of some drinking waters supplying the city of Yerevan. Proceedings of NAS RA, Earth Sciences LVIII(1): 41 to 45. GH 2.9 to 10.9, pH 6.01 to 7.30, and the Hrazdan catchment geology including the carbonate outcrops. arar.sci.am PDF
  20. Gazanik category feeds and product pages. The shop's own published catalogue and its live category pages, 8 and 9 September 2026. The 16 stones, the single driftwood listing at 29,000 AMD, and the 2,200 to 3,000 AMD per kilogram band. Individual prices are indicative. gazanik.am
  21. Our own arithmetic. Not a published source. The carbonate ceiling and every figure derived from it. Method: the standard calcite carbonate equilibrium system solved at 25 degrees C for a fixed dissolved CO2, charge balance 2[Ca2+] = [HCO3-], using log K1 -6.35, log K2 -10.33 and log Ksp(calcite) -8.48, activity corrections ignored. Constants from sources 1 and 2, validated against the calcite solubility figures on page 92 of source 1. calculated, repeatable, unverified by a third party
  22. the stone and wood tables with their per row confidence ratings; Armenian stone, and 2.5, Yerevan tap water; ; Each carries its own numbered source list to the primary material. The hub's sources page

Continue

Where to go next.

Find a hardscape

The finder, with the ceiling argument and the vinegar test written out.

Materials and water

What a carbonate rock actually does to a tank, and the saturation ceiling that caps it.

Armenia is standing on its hardscape

Armenian tuff, scoria and obsidian, what each one does to the water, and the bucket test that settles any rock in a fortnight.

Forgiving to demanding

For hardscape, forgiving means inert and known, scored here as two separate conditions.

Phosphate, iron, copper and tannins

Iron, phosphate, copper and tannins, which is what the leaching column is actually about.