Hardscape Chemistry Guide — Rocks, Wood & Your Water
"I added a rock and my water chemistry went sideways" is one of the most common aquarium mysteries. Every material below is profiled the same way the AquaLens app profiles it when diagnosing parameter drift in your tank.
Limestone-based — dissolves slowly and pushes KH, GH, and pH up over weeks, even though it's an aquascaping favorite.
KH, GH, pH up
Hard-water and rift-lake tanks; risky in soft-water, planted, or blackwater setups
Strongly calcareous limestone — the classic cichlid rock precisely because it keeps KH, GH, and pH high.
KH, GH, pH up
African rift-lake cichlid tanks
Often sedimentary and mildly calcareous — can raise KH, GH, and pH. Worth an acid test before use in a soft-water tank.
KH, GH, pH up (mildly)
Hard-water tanks; vinegar-test before trusting it in soft water
Clay-based and generally inert — little effect on hardness or pH. Crumbly, so rinse well to avoid clouding.
None expected
Planted and shrimp tanks
Usually inert volcanic rock, though some pieces are calcareous. Acid-test if you're unsure.
Usually none — verify
Most freshwater tanks once acid-tested
Inert and very porous — no chemistry effect, and the surface area helps beneficial bacteria.
None expected
Any tank; porosity aids biological filtration
Inert — no measurable effect on water chemistry.
None expected
Any tank, including soft-water and planted
Inert silica-based stone — no effect on hardness or pH.
None expected
Any tank, including soft-water and planted
Calcium-carbonate based, but its buffering is expected and managed in a reef — the app tracks it for weight and biological maturity, not as a chemistry anomaly.
Part of normal reef buffering
Reef and saltwater tanks
Reef base rock. Cured dry rock is effectively inert; uncured rock can leach phosphate while it matures.
Cured: none; uncured can leach phosphate
Reef and saltwater tanks
Leaches tannins and organic acids that tint the water and can lower pH, especially in soft water.
pH down, tannin tint
Planted, blackwater, and soft-water tanks
Heavy tannin release — expect noticeable tinting and a downward nudge on pH until it's leached out.
pH down, strong tannin tint
Blackwater and tannin-tolerant tanks; pre-soak recommended
Releases tannins and mild acids that can lower pH in soft water.
pH down, tannin tint
Planted, blackwater, and soft-water tanks; sinks readily
Relatively low tannin, but still leaches some acids that can nudge pH down early on.
pH down slightly, low tannin
Aquascapes where minimal tinting is wanted
Soft wood that breaks down over time, releasing tannins and mild acids — popular with shrimp keepers.
pH down, tannins; slowly breaks down
Shrimp tanks — grazing surface and shelter
Most aquarium-safe resin is inert. Cheap or non-aquarium castings can leach, so acid-test and watch your parameters after adding it.
None if aquarium-safe
Any tank, if aquarium-grade
The vinegar-fizz test
Drip plain white vinegar (or, more decisively, a few drops of a stronger acid like the API pH-down reagent) onto a dry spot on the rock. If it fizzes or bubbles, the rock contains calcium carbonate: acid plus carbonate releases CO₂ gas. A fizzing rock will do the same thing slowly in your aquarium — the mild acidity of tank water continuously dissolves a little of it, releasing carbonates that push KH up, minerals that push GH up, and, through the extra buffering, pH up with them. That's exactly why Texas holey rock is prized in a Malawi cichlid tank and unwelcome in a soft-water shrimp tank. The app treats a fizzed test as physical evidence that overrides an "inert" label, and this guide follows the same logic: when in doubt, test the actual piece.
Driftwood works in the opposite direction. Wood leaches tannins and other organic acids that tint the water amber and nudge pH downward — noticeably in soft, low-KH water, barely at all in hard water where the buffering absorbs it. The tint is harmless (many blackwater fish prefer it) and fades over weeks as the wood leaches out; pre-soaking or boiling smaller pieces speeds that up. The practical habit that prevents every one of these mysteries: test KH, GH, and pH before adding a new piece, then again a week or two later, so any drift has an obvious suspect.