Reef tank: fine sand, coarse sand, or bare bottom?
Three options exist for the bottom of a reef tank: fine sand (under 1 mm), coarse sand or crushed coral, and bare bottom (no substrate at all). None is universally best: the right choice mostly depends on which fish you keep and how much maintenance you are willing to do.
Does sand actually change fish behaviour?
Yes, and not only for burrowing species. A 2023 study in the journal Animals by Maria Mafalda Almeida, Elsa Cabrita and Elvira Fatsini compared juvenile sole kept with or without a sand substrate: the presence of sand changed dominance behaviour, reduced aggressive interactions near the feeding point, and was accompanied by differences in the expression of stress-related brain genes (study on Google Scholar). Substrate choice is therefore not purely cosmetic: it directly influences the welfare of fish that interact with the bottom.
Does fine sand really reduce nitrates?
This is the classic argument for a deep sand bed: the oxygen-poor lower layers are supposed to host denitrifying bacteria that convert nitrate into nitrogen gas. The underlying biogeochemical mechanism does occur in nature — work led by Charles Schutte and colleagues at the Max Planck Institute for Marine Microbiology, published in 2021 in Frontiers in Microbiology, describes how water advection through sandy sediment fuels denitrification at depth (study on Google Scholar). In a closed tank the effect stays modest compared with regular water changes, and it never replaces tracking the causes of high nitrates.
Why do some hobbyists choose bare bottom instead?
A bare bottom makes cleaning much easier: nothing traps waste, and a simple daily siphon is enough. It is the preferred choice for a quarantine tank, or for species that do not dig, such as some clownfish or damsels in a simple community setup.
- Fine sand: needed for burrowing gobies and wrasses that bury themselves at night
- Coarse sand: a good visual compromise, less easily stirred up by flow
- Bare bottom: easiest to maintain, but uncomfortable for burrowing species
- A minimum depth of 5-8 cm if the goal is natural denitrification
For the full range of substrate hardware, see substrate for your aquarium, and our habitat and aquarium hub.
Frequently asked questions
Can fine sand and live rock be combined?
Yes, and it is the most common reef setup: sand on the bottom, live rock structured above it, without resting rock directly on very fine sand that could shift and collapse.
Does bare bottom suit a clownfish?
Yes, clownfish do not dig into the substrate and adapt well to a bare bottom, unlike a cleaner wrasse or a sand-sifting goby that genuinely need it.
Does a reef tank need a specific type of sand?
Aragonite sand is recommended: it naturally buffers pH, unlike inert silica sand, which suits freshwater setups better.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.