Reef Salt Mix True Cost Per Gallon

Salt brand · RO/DI membrane cost · Waste-water ratio · Water change schedule → real $/gallon and annual budget

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Tank & Water Change

Display tank volume minus rock/sand displacement. Typical 75 gal display = ~65 gal net.
Common reef practice: 10–20% weekly or 20–25% bi-weekly.
Weekly = 4.3 · Bi-weekly = 2.15 · Monthly = 1

Target Salinity

Reef standard is 35 ppt (SG 1.0264 @ 25 °C). NSW = 35 ppt.
Used to estimate DI resin exhaustion. Find via your utility's water report or a TDS meter.

RO/DI System

Standard 75-GPD membrane ≈ 3:1. High-efficiency ≈ 1:1. Enter e.g. 3 for 3:1.
US average ≈ $0.007–0.012/gal combined water+sewer. Enter 0 if on well.
Price paid for replacement membrane.
Typical 75-GPD membrane: 1000–2000 gal product water before TDS creep.
Price for DI resin refill (color-change resin, per canister).
Exhaustion depends on source TDS. High-TDS water exhausts resin faster. Typical range: 100–500 gal.
Combined cost for sediment + carbon block replacement set.
Pre-filters typically rated 2000–6000 gal product water.

Salt Brand & Cost

Choose how to enter your salt quantity.
e.g. Red Sea Coral Pro 22 lb bucket = 352 oz; Reef Crystals 50 lb = 800 oz.
Include shipping if ordering online.
Results

How to Use This Calculator

Enter your tank's net water volume, water change percentage, and how often you change water each month. Then fill in your salt brand details (weight per bucket and price paid, or labeled gallon yield and price) and your RO/DI system parameters. The calculator instantly shows your true cost per gallon of new saltwater, cost per water change, and monthly and annual totals.

  1. Tank & Water Change — Enter net water volume (subtract rock/sand displacement from display volume), water change size as a percentage, and changes per month.
  2. Target Salinity — Standard reef target is 35 ppt. Enter your tap TDS so the DI resin life estimate adjusts to your water quality.
  3. RO/DI System — Enter your waste ratio (check your membrane spec sheet), tap water + sewer rate (find this on your utility bill), and costs and rated lives for your membrane, DI resin, and pre-filters.
  4. Salt Brand — Choose whether to enter salt by weight (oz or grams) or by labeled gallon yield, then enter the price paid including any shipping.
  5. Results update instantly. Use Print / Save PDF to archive your numbers, or Copy Results to paste into a forum post for comparison.

Why "True Cost" Matters

Most reefers compare salt prices by bucket cost alone and miss the full picture. Three often-overlooked costs add up fast:

Formula & Method

Gallons of new saltwater per change

New saltwater = tank volume (gal) × (water change % ÷ 100)

Salt required per change

At target salinity S (ppt), 1 gallon of water weighs ≈ 3.785 kg (8.345 lb). Salt required = S × 3.785 g per gallon of new saltwater. (At 35 ppt: ≈ 132.5 g ≈ 4.67 oz per US gallon.)

RO/DI water drawn per change

Total tap draw = new saltwater volume × (1 + waste ratio). E.g. making 15 gallons at 3:1 waste = 60 gallons drawn from the tap.

Filter costs prorated per change

Membrane cost per gallon of product = membrane price ÷ rated life (gal). Similarly for DI resin and pre-filters. These are multiplied by the gallons produced per change and summed.

Total cost per gallon of new saltwater

Sum of: (salt cost per gallon) + (tap water cost × tap draw per gallon of product) + (membrane cost per gallon) + (DI cost per gallon) + (pre-filter cost per gallon).

Source: salinity–SG relationship per Sea-Bird Scientific Practical Salinity Scale; salt mass per gallon from standard aqueous solution chemistry.

Frequently Asked Questions

Why does the cost per gallon of reef saltwater vary between salt brands?
Different brands sell salt in different container sizes at different prices, and with slightly different mix ratios. A bucket that mixes "200 gallons" may cost the same as one mixing "160 gallons"—the per-gallon price can differ by 30–40%. This calculator lets you enter the exact package weight (or yield) and price so you can compare apples-to-apples across brands.
What is the RO/DI waste ratio and why does it matter for cost?
A reverse-osmosis/deionization (RO/DI) system pushes tap water through a membrane; only a fraction becomes purified product water — the rest exits as waste. A standard 75-GPD membrane at typical municipal pressure has a waste ratio of roughly 3:1 to 4:1 (3–4 gallons wasted per gallon produced). High-efficiency membranes can reach 1:1. If your water utility charges even $0.008/gallon including sewer, making 20 gallons of RO/DI at 3:1 means drawing 80 total tap gallons — a cost that compounds significantly over a year.
How much salt do I need per gallon at SG 1.025?
At 35 ppt (≈ SG 1.0264 at 25 °C), you need approximately 132–133 g (about 4.67 oz) of salt per US gallon of new saltwater. This assumes you are making freshly mixed saltwater to replace the old. This is the formula used by the calculator (S × 3.785 g/gal, where S is your target ppt).
How often should I change water in a reef tank?
A widely practiced approach is 10–20% weekly or 20–25% bi-weekly. High-nutrient or heavily stocked SPS tanks may need more frequent changes; lightly stocked softie tanks with good nutrient export can sometimes go 2–4 weeks. The most important factor is consistency — abrupt large changes cause more stress to corals than smaller, regular ones.
Should I count RO/DI filter costs in my water-change budget?
Yes. A typical 4-stage RO/DI system uses sediment, carbon block, RO membrane, and DI resin. The membrane is often rated for 1,000–2,000 gallons before TDS creep, while DI resin may exhaust at 200–400 gallons depending on source-water TDS. Prorating these costs reveals that in hard-water areas the per-gallon filter cost can equal or exceed the salt cost — and is routinely overlooked in "which salt is cheapest" comparisons.
What is the difference between specific gravity and salinity (ppt)?
Salinity in parts per thousand (ppt or ‰) measures grams of dissolved salt per kilogram of water. Specific gravity (SG) is the ratio of the water's density to pure water at the same temperature. At 25 °C, reef-target salinity of 35 ppt corresponds to SG ≈ 1.0264. Swing-arm hydrometers read SG directly; refractometers calibrated for seawater typically read ppt. The relationship: ppt ≈ (SG − 1) × 1000 × 1.4 at normal reef temperatures (~25 °C).