Seachem Onyx Gravel 7 Kg

Seachem Onyx is a premium, carbonate-rich natural substrate ideal for Tanganyikan cichlids and other hard-water freshwater setups.

• Premium natural substrate for the aquarium.
• Ideal for Tanganyikan Cichlids.
• Rich in carbonates.

Seachem Onyx is a specially fracted natural substrate for the aquarium. Seachem Onyx is suitable for any marine or freshwater environments and is ideally suited for Cichlid environments. Seachem Onyx is most effective when used alone as an integral substrate bed, but it may be mixed with other gravels or sands such as Onyx San. Onyx is not chemically coated or treated but does have a slight buffering capacity.

The Seachem Onyx Gravel 7 Kg forms the base layer of a planted tank — see our guide to Substrate & Aquasoil for how to pick and layer substrate for healthy root growth.

Specifications

Seachem item #3705 (7 kg). A specially fracted natural substrate, suitable for marine or freshwater and aimed squarely at rift-lake cichlids. It is not chemically coated or treated, but it does have a slight buffering capacity — which in a cichlid tank is the point rather than a side effect.

Seachem publish the mineral analysis. Units are mg/kg; the method is a plasma emission spectrometer, EPA Method 6010, run by the University of Georgia Chemical Analysis Laboratory:

The calcium and magnesium figures are the reason it buffers, and the reason it suits hard-water cichlids.

How far a pH shift to expect: Seachem do not say for this product. They publish a figure of 0.1–0.5 pH units for Onyx Sand, but not for Onyx gravel, and we are not going to move a number between two different products. Test your water rather than assume it.

Setup & Use

Use it as the whole substrate rather than as a layer under gravel. Seachem say it is most effective used alone as an integral bed, though it may be mixed with other gravels or with Onyx Sand.

Rinse it first. It is pre-washed, but it is a natural product and it picks up dust in transit. Some fines will remain however long you rinse. That is normal and costs the tank nothing beyond a week of cleaning the filter more often than usual.

Fill the tank through a bowl. Stand a bowl on the substrate and pour into the bowl so the water overflows softly onto the bed rather than cratering it. Initial cloudiness is normal and clears with mechanical filtration over a day or two; Clarity accelerates it.

No gravel modifier is needed. Laterite and the like are not required underneath it.

Coverage: Seachem run a project calculator on their own site that converts tank length, width and intended depth into a number of 7 kg bags. Work it out before you order — running one bag short halfway through a rescape is a bad afternoon.

Freshness & Storage

Every batch we dispatch carries a minimum of 6 months' remaining shelf life. Store this product in a cool, dry place away from direct sunlight, with the cap or inner foil closed tightly after each use.

Filter media and substrates carry no manufacturing or expiry date — their working life begins on first contact with water, not on the shelf.

Returns & Replacement

Returns are accepted within 7 days of delivery, provided the item is unused and in its original packaging. Manufacturer-sealed items cannot be returned once the seal is broken, and sale-priced items are not refundable.

If your item arrives defective or damaged, message us on WhatsApp with your order number — or email us, if you would rather — and we will replace it. Approved replacements are delivered within 3 to 5 business days. Transit damage is covered by insurance on every shipment. If tracking shows delivered but nothing reached you, tell us within 72 hours, which is the reporting window Indian couriers allow. Damage caused during installation or use is not covered; replacement spares for such cases are chargeable.

Full terms are on our Cancellation & Returns page.