How many pounds of sand do I need for a 75-gallon reef tank?

A 75-gallon (48 × 18 × 21 in) needs about 60–70 pounds for a 1-inch bed and 90–105 pounds for a 1.5-inch bed, depending on grain choice. The calculator gives the exact number for the product you select.

What's the difference between aragonite and silica sand?

Aragonite (CaCO₃) is the reef-standard substrate — pH-buffering, calcium-releasing, and matches natural seawater chemistry. Silica (SiO₂) is inert pool-filter sand, cheap, and totally fine for freshwater. Silica won't release silicate into a healthy tank (the diatom-bloom myth is largely debunked), but it doesn't help buffer either.

How thick should my sand bed be?

1–1.5 inches is the modern reef sweet spot — enough to look natural and host pods, not deep enough to trap detritus. Skip 2–3.5 inches entirely (gets the worst of both worlds). 4+ inches if you're committing to a deep sand bed setup with proper infauna.

Live sand vs dry sand?

Dry sand is cheaper and lighter to ship. Live sand (CaribSea Arag-Alive) is bagged wet with a bacterial inoculant — marginally faster cycle start, but bacteria from a working tank or a dose of Dr. Tim's One & Only will catch up within a week.

Will sand blow around in high flow?

Sugar-sized oolite will blow at MP40-level flow if aimed at the sand bed. Aim powerheads horizontally or up. If you can't avoid it, step up to Special Grade Reef (1–2 mm) which is heavy enough to hold position.

Calculator

Aquarium Sand Calculator

"Sand bed" covers four very different setups — bare-bottom, shallow sand bed (SSB, <1.5 in), deep sand bed (DSB, 3+ in), and the unstable middle zone you should skip. The calculator handles the math; the H2 sections below tell you which setup belongs in your tank.

Substrate

Tank length (in)

Tank width (in)

Bed depth (in)

1–2 in is the common shallow-bed range. 3+ in is a deep sand bed.

Substrate product

Substrate needed

63.8lb

Metric

28.9kg

Volume

0.75cu ft

Bags (20 lb)

4bags

Bag density: 85 lb/ft³

Density source: caribsea.com

How this is calculated

cubic_inches = length_in × width_in × depth_in
cubic_feet   = cubic_inches / 1728
pounds       = cubic_feet × lb_per_cuft   // by product

Bare-bottom, SSB, DSB — which to pick

Bare-bottom reef tanks (no sand at all) are popular among SPS-focused systems where high flow and zero detritus traps matter more than the natural look. Easy to siphon, no anoxic risk, but coral on the sandbed looks weird and you lose burrowers like wrasses and gobies.

Shallow sand bed (SSB) — 0.5 to 1.5 inches of fine aragonite (sugar-sized or Special Grade Reef). This is the modern reef standard. Low detritus risk, supports wrasses and gobies, no anoxic chemistry to worry about. Stir gently with a turkey baster on water-change day.

Deep sand bed (DSB) — 4+ inches of fine sand. The lower layer goes anoxic and supports denitrifying bacteria, dropping nitrate passively. Powerful when it works; nuclear when it crashes after years of accumulated detritus. Commit fully or skip it (Shimek's DSB reviews are the standard reference).

Skip the 2–3.5 inch range. Too deep to oxygenate fully, too shallow to denitrify reliably. You get hydrogen sulfide pockets without the nitrate-reduction benefit.

Grain size matters

Sugar-sized oolite (0.25–1 mm) packs densest and looks like a beach. It's also the easiest to blow around with strong flow — keep powerheads aimed up. Special Grade Reef (1–2 mm) is the middle ground: stays put, but coarser-looking. Bimini Pink and Hawaiian Black are the same density band with cosmetic variety. Tahitian Moon Sand (Estes) is a popular freshwater black option at similar density.

FAQ

How many pounds of sand do I need for a 75-gallon reef tank?: A 75-gallon (48 × 18 × 21 in) needs about 60–70 pounds for a 1-inch bed and 90–105 pounds for a 1.5-inch bed, depending on grain choice. The calculator gives the exact number for the product you select.
What's the difference between aragonite and silica sand?: Aragonite (CaCO₃) is the reef-standard substrate — pH-buffering, calcium-releasing, and matches natural seawater chemistry. Silica (SiO₂) is inert pool-filter sand, cheap, and totally fine for freshwater. Silica won't release silicate into a healthy tank (the diatom-bloom myth is largely debunked), but it doesn't help buffer either.
How thick should my sand bed be?: 1–1.5 inches is the modern reef sweet spot — enough to look natural and host pods, not deep enough to trap detritus. Skip 2–3.5 inches entirely (gets the worst of both worlds). 4+ inches if you're committing to a deep sand bed setup with proper infauna.
Live sand vs dry sand?: Dry sand is cheaper and lighter to ship. Live sand (CaribSea Arag-Alive) is bagged wet with a bacterial inoculant — marginally faster cycle start, but bacteria from a working tank or a dose of Dr. Tim's One & Only will catch up within a week.
Will sand blow around in high flow?: Sugar-sized oolite will blow at MP40-level flow if aimed at the sand bed. Aim powerheads horizontally or up. If you can't avoid it, step up to Special Grade Reef (1–2 mm) which is heavy enough to hold position.

Share / embed this calculator

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  width="100%"
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Sources & references

01
CaribSea — Marine Substrates (lb/ft³ tables)
https://caribsea.com/marine-substrates/
02
Ron Shimek — DSB methodology reviews (Reefkeeping)
https://reefkeeping.com/issues/2002-10/rs/index.php
03
Randy Holmes-Farley — Aquarium Chemistry: Aragonite
https://reefkeeping.com/issues/2002-09/rhf/index.php

Last reviewed 2026-05-25