Pumice as Aquarium Substrate: A Guide
How porous volcanic pumice is used as a lightweight base layer beneath nutrient soils in planted aquariums, and what its structure offers bacteria.
Overview
Pumice is a volcanic rock formed when super-heated, highly pressurised rock is rapidly ejected from a volcano. Simultaneous rapid cooling and depressurisation cause dissolved gases to escape, freezing a foamy network of bubble cavities into the rock. In aquariums this porous mineral is used as a lightweight base layer placed beneath nutrient-rich aqua soils.
What it is
Pumice is composed mainly of highly microvesicular volcanic glass, typically felsic to intermediate in composition (rhyolitic, dacitic or andesitic). It is light-coloured, ranging from white and grey to blue, green-brown or black. Its defining feature is an extremely high proportion of air-filled vesicles.
Physical properties
- Porosity: roughly 64-85% by volume
- Density: specimens can be around 0.25 g/cm3
- pH neutral; does not decompose or compact significantly
- Most pumice floats on water until it becomes waterlogged
Because the vesicles are initially filled with air, most pumice floats on water, sometimes for years, until water gradually penetrates the cavities and the rock sinks. This behaviour is relevant when first submerging fresh material.
Why it suits substrates
The same porosity that makes pumice useful in horticulture applies underwater: its porous structure allows water and gas transport, it is pH neutral, and it resists compaction. As a base layer it lifts a nutrient soil cap and reduces packing in deep planted layouts.
Bacterial habitat
Biological filtration in an aquarium relies on a chemically inert porous surface that provides a large area for nitrifying bacteria such as Nitrosomonas, Nitrospira and Nitrobacter to colonise. A porous mineral layer offers extensive internal surface area on which these bacterial colonies can develop within the substrate.
Practical notes
- Rinse before use to remove dust
- Expect fresh pieces to float until waterlogged
- Use as an underlayer beneath, not on top of, a nutrient cap
- Its low density makes it easy to handle in deep layouts