Aquatic Roman Gravel Jet Black

A fish tank's substrate does two jobs at once: it is the backdrop every fish in the tank swims in front of, and it is also where a good deal of the tank's biology quietly carries on beneath the surface.

This is Pettex's Roman gravel in jet black, sold in a 2kg bag. It is made from selected silicate aggregate, chosen for being chemically inert, and coated in a colourfast resin so the black stays black rather than fading or leaching into the water. Pettex say it will not change the water's pH, and its fine grain size means it can double as a planting medium: the increased surface area supports the anaerobic activity a planted tank relies on.

• 2kg bag of decorative aquarium gravel
• Colour: jet black, coated in a colourfast resin
• Made from silicate aggregate, chosen for being chemically inert
• Pettex state it will not change the water's pH
• Suitable for coldwater, tropical and saltwater set-ups
• Fine grain size, usable as a planting medium

What the label does not say. There is no stated grain size in millimetres, so if you are trying to match an existing substrate exactly, this description will not get you there. Pettex do not publish a per-litre coverage guide either, so working out how many bags a given tank needs is left to you.

Who it's for: anyone rescaping a coldwater, tropical or marine tank who wants one black gravel that works across all three, rather than buying a different bag for each.