Scope: decorative/functional in-tank items that aren't the tank, stand, filtration, heating, or lighting — substrate (gravel/sand/soil), hardscape (rock/driftwood), ornaments, aquarium plants (live or artificial), and backgrounds, across freshwater and marine tanks.

## Seed
1. Aquatic-decor equivalent of harvest date: water-chemistry reactivity of the rock/substrate material (calcium-carbonate reactive vs inert) — rarely stated on a listing, but it silently raises pH/hardness for months and can stall soft-water fish or plants, the way a stale harvest date silently kills an oil's flavor.
2. A reef/water-chemistry-minded keeper notices first whether a rock would fizz in vinegar (reactive calcium carbonate) before noticing its color or shape — the typical buyer picks decor by looks and price.

## Sources
[wikipedia_aquascaping] https://en.wikipedia.org/wiki/Aquascaping — Iwagumi/Dutch/Nature-style hardscape conventions; driftwood/botanicals release tannins that darken and soften/acidify blackwater tanks — fetched excerpt
[wikipedia_calcium_carbonate] https://en.wikipedia.org/wiki/Calcium_carbonate — CaCO3 reacts with acid, releasing Ca2+, CO2, water — fetched excerpt
[aquariumcoop_substrate] https://www.aquariumcoop.com/blogs/aquarium/aquarium-substrate — inert (CEC, no pH change) vs active (lowers pH/softens, breaks down 1-2yr) substrate; avoid active soil for African cichlids; grain-size extremes hinder roots — fetched excerpt
[seachem_flourite] https://www.seachem.com/flourite.php — Flourite is a porous clay gravel, not chemically coated, will not alter pH — fetched excerpt
[fluval_stratum] https://web.archive.org/web/20220517211433/https://fluvalaquatics.com/us/stratum-series/ — Stratum is a volcanic nutrient soil for live plants; brand's own decor taxonomy (driftwood natural/polyresin, substrate, ornaments, backgrounds, decor plants) — fetched excerpt via Wayback (live site is Cloudflare-blocked to curl)
[caribsea_home] https://www.caribsea.com/ — brand nav splits Marine Substrates / Marine Rocks from Freshwater Substrates / Freshwater Rocks — fetched excerpt
[tropica_guide] https://tropica.com/en/guide/make-your-aquarium-a-success/ — Tropica's patented Easy/Medium/Advanced difficulty symbols printed on plant-pot labels — fetched excerpt

## Branches
- water_type -> marine_reef: CaribSea's own catalog separates Marine Substrates/Rocks from Freshwater Substrates/Rocks, i.e. the brand itself treats material choice as gated on water type.
- decor_type -> 5 values: Fluval's own site nav buckets decor into exactly driftwood / substrate / ornaments / backgrounds / decor plants, independently corroborated by CaribSea's parallel substrate/rock split.
- plant_type -> live vs artificial: wikipedia_aquascaping describes live-plant aquascaping conventions (Iwagumi, Dutch, epiphyte moss) that don't apply once a plant is artificial; Fluval catalogs "Decor Plants" as a distinct bucket from live planted-tank substrate lines.

## Noise
- CaribSea nav row "Water/Additives" — filtration/water-conditioner equipment, belongs to the aquatic-equipment sibling leaf.
- CaribSea nav row "Reptile" — terrarium products, belongs to the terrariums sibling leaf.
- Fluval nav row "Maintenance" (scrubbers, gravel cleaners, fish nets) — belongs to the aquatic-equipment sibling leaf.
- Fluval nav row "Aquariums & Furniture" — the tank/stand itself, belongs to the aquariums sibling leaf.
- Merch/apparel rows on both brand sites — not aquarium products.
- No regulator or test standard governs aquarium decor materials (unlike CPSC/ASTM toy standards, which don't apply to pet-only items); water-chemistry reactivity is carried as sourced facets/a judgement instead of a certification.
- Considered an "ornament edge safety / fin-nipping" facet; dropped it rather than assert it on the one weak, non-corroborating source reachable this run.
- Considered a separate "plant material softness" facet; dropped it as redundant with plant_type's own artificial_silk/artificial_plastic values.

## Corrections
- Validator: reference "kind" must be "specialist_review" not "specialist_reviewer"; fixed both refs.
- Validator: split/cold-start questions can't start with "what kind of"; reworded decor_type's question.

## Corrections (2026-10-26)
- Removed water_type->marine_reef's then_ask override that bypassed decor_type; branch now only drops plant care facets.
- Removed aesthetic_style from water_type->marine_reef's makes_irrelevant; biotope_correct is a real marine/reef style.
- Added hardscape_piece_size facet (small/medium/large), cited to caribsea_dragon_arches + caribsea_catalog (8"/12"/18" and accent-piece sizing).
- Added hardscape_porosity facet (high/moderate/dense), cited to caribsea_catalog + wikipedia_live_rock (surface area for nitrifying bacteria, distinct from calcium reactivity).
- Added substrate_color facet, cited to seachem_flourite (Red/Black/Dark SKUs) + caribsea_catalog (Super Naturals color variants).
- Added vocabulary entries: Porosity, Nitrifying bacteria, Bioload.
- Reclassified substrate_grain_size and hardscape_prep_required from source:listing to source:researched; their own backfill_note/listing_field_hint already admitted they're not on most listings.
- Added evidence (caribsea_catalog, seachem_flourite) to substrate_cost_per_liter and grounded its "why" in sourced bulk-density figures: CaribSea spec sheets list 54-100 lb/ft3 across substrate lines; Flourite's bag-volume spec implies ~1.27 kg/L, consistent with that range.
- Wired new facets (hardscape_piece_size, hardscape_porosity, substrate_color) into decor_type's branches' makes_irrelevant lists for consistency with existing companion facets.