Seachem Onyx sand 3.5kg
Water type
Freshwater
Seachem Onyx sand 3.5kg
A carbonate-rich, clay-based substrate that gently buffers pH while your plants root into it. Seachem Onyx Sand is a natural, uncoated substrate built for freshwater planted aquariums. It's dark grey rather than black, porous enough for roots to establish quickly, and because it's mineral rather than chemically treated, it never needs replacing — set it up once and leave it in place for the life of the tank.
Key features
- Carbonate-rich composition — naturally raises pH by roughly 0.1 to 0.5 units, depending on your source water, without a separate buffer.
- No chemical coating — it's a natural clay-based mineral, not a dyed or resin-coated gravel.
- Permanent substrate — never needs replacing or refreshing over the life of the aquarium.
- Porous structure — supports strong root development in heavily planted layouts.
- Flexible use — works as a standalone substrate or as a nutrient-buffering base layer under a finer cap.
- Dark grey tone — sits neutrally behind planted layouts without the stark black of dyed substrates.
Specifications
| Brand | Seachem |
|---|---|
| Product | Onyx Sand |
| Pack size | 3.5 kg |
| Material | Natural carbonate-rich clay substrate, uncoated |
| Water type | Freshwater |
| pH effect | Raises pH by approximately 0.1–0.5 units, depending on source water hardness |
| Colour | Dark grey |
| Replacement interval | Not required — permanent |
| SKU | AA0449 |
| Barcode | 116350303 |
Which size do I need?
Onyx Sand is sold in a 3.5 kg bag and a larger 7 kg bag. Seachem doesn't publish an exact coverage depth per kilogram, so the right choice comes down to your tank's footprint: the 3.5 kg bag suits a nano to small planted tank, or works as a thin buffering base layer under a finer cap in a larger tank. If you're not sure how far a bag will go in your specific tank, get in touch before ordering rather than guessing — it's cheaper to check than to reorder.
Suitable for, and not suitable for
- Suitable for: freshwater planted aquariums, aquascapes wanting a natural buffering base layer, tanks with softer source water that benefit from a mild upward pH nudge.
- Not suitable for: marine or brackish systems, tanks where you need a completely inert substrate with zero pH effect, or setups needing a bright white or pale substrate for contrast.
Care and maintenance
Onyx Sand is pre-washed at the factory but can pick up dust in transit, so rinse it before adding it to the tank. Once planted, avoid heavy stirring or deep gravel vacuuming around root zones — light surface cleaning is enough, since the substrate itself never needs replacing or topping up.
Warranty and delivery
As a natural mineral substrate, Onyx Sand doesn't carry a manufacturer warranty the way equipment does — Seachem's guarantee is on product quality at the time of sale, in addition to your rights under Australian Consumer Law. It ships as a standard parcel with no special freight handling required.
Common questions
Will Onyx Sand keep raising my pH indefinitely?
No. The buffering effect is mild and depends on your source water's existing hardness — it nudges pH upward rather than driving it to a fixed target the way a dedicated buffer product does.
Can I use Onyx Sand in a saltwater or brackish tank?
No. Onyx Sand is formulated and sold for freshwater planted aquariums only.
Do I need to rinse it before use?
Yes. It's pre-washed by Seachem but can become dusty in transit, so a rinse before adding it to the tank will keep your water clearer during setup.
Can I mix Onyx Sand with other substrates?
Yes — many aquascapers use it as a base layer under a finer-grained cap substrate, taking advantage of its buffering properties without it being the visible top layer.
Does it need replacing over time?
No. It's a natural, uncoated mineral substrate that Seachem states is good for the life of the aquarium.
