Saltwater vs freshwater: what the same tank costs each month

Same 40-gallon tank, same heater, same room. Set up as freshwater, the example below costs about $9.81 a month to run. As a reef, it's about $33.81, roughly 3.4 times as much. Salt, pumps and RODI water make up most of the gap. Every value is editable, and your inputs stay on this device.

Shared inputs

Electricity rate ($ per kWh) Heater wattage (W, both tanks) Heater duty cycle (%) Water + sewer rate, $ per 1,000 gallons Water change, gallons per week Top-off (evaporation), gallons per day

Freshwater

Light wattage (W) Light hours per day Filter wattage (W, runs 24 h) Water conditioner bottle price ($) Bottle size (mL, dosed 5 mL per 50 gal) Test strips, price per pack ($) Strips per pack Tests per week

Saltwater reef

Light wattage (W) Light hours per day Return pump (W, 24 h) Powerheads, total (W, 24 h) Skimmer (W, 24 h) Salt price per box or bucket ($) Gallons that box or bucket mixes RODI water cost, $ per gallon Test kit price ($) Tests per kit Tests per week

Freshwater, monthly

Saltwater, monthly

Saltwater costs more per year by

Line	Freshwater	Saltwater	Difference

Formulas: kWh/month = watts × hours/day × duty × 30.44 ÷ 1000. Monthly gallons = weekly gallons × 52 ÷ 12, and top-off = gallons per day × 30.44. Freshwater water cost is tap water (water changes plus top-off) at the water + sewer rate; saltwater uses RODI water at its cost per gallon for both. Conditioner = water-change gallons × 5 mL ÷ 50 × price ÷ bottle mL. Salt = water-change gallons × price ÷ rated gallons. Food, additives and filter media are left out of both sides because they vary too much by livestock to compare fairly; add them in the full running-cost calculator.

Where the examples come from: 18¢/kWh is close to the US average residential price of 18.31¢ for July 2026 (Colorado: 17.00¢), from the EIA Electric Power Monthly, Table 5.6.A. Both tanks use the same heater: Fluval's M150 is 150 W and rated for tanks up to 45 gallons (Fluval); the 30% duty cycle is inside the 20–40% range ReefKalk gives for heaters in a heated room. Freshwater: the Fluval Aquasky 2.0 for 36–48 inch tanks is 27 W (Fluval), and the Fluval AC70 hang-on-back filter, rated for 40–70 gallons, is 7 W (Fluval). Seachem Prime is dosed at 5 mL per 50 US gallons (Seachem); a 500 mL bottle was $18.99 and API 5-in-1 test strips (100 count) were $24.98 at Bulk Reef Supply on October 17, 2026. $16.83 per 1,000 gallons is EPA WaterSense's 2024 US average residential water plus wastewater rate. Saltwater: the AquaIllumination Prime 16HD lists 59 W at full power and a 24 × 24 inch spread (AI); the Sicce Syncra SDC 3.0 return pump is rated 6–35 W (Sicce); the EcoTech VorTech MP10mQD powerhead is rated 8–19 W for 2.5–50+ gallon tanks (EcoTech); the Reef Octopus Classic 110-INT skimmer pump draws 9 W on 60 Hz US power (Reef Octopus). We use the maximum ratings, so the reef side is a high estimate. BRS recommends a 10% weekly water change, which it puts at 5 gallons for a 40-gallon tank (BRS), and we use the same 5 gallons for the freshwater tank so the comparison is like for like. BRS says a 40-gallon tank evaporates about half a gallon a day (BRS); we assume the same for both tanks. Salt: Instant Ocean Sea Salt, $89.99 for a box that makes 200 gallons at SaltwaterAquarium.com on October 17, 2026. RODI cost is the example result from our RODI calculator. Testing: the Salifert kH/Alkalinity kit, $18.79 at Bulk Reef Supply on October 17, 2026, which BRS says gives 100–200 tests, "typically closer to 100". Prices change, so enter what you pay.

Why saltwater costs more to run

Salt is a whole new line. Freshwater changes use tap water and a few cents of conditioner. A reef pays for salt on every gallon it changes: about $9.75 a month in this example.
Reefs need moving water. A return pump, powerhead and skimmer run around the clock. Even at modest wattage, that's the biggest electrical difference.
RODI water instead of tap. A reef uses purified water for both changes and top-off. It's cheap per gallon but costs about 10 times as much as tap water at these rates.
The heater is a wash. At the same tank size and temperature, both tanks pay about the same to heat, so it doesn't add to the gap.
Lighting is a small gap. A reef light draws more, but LEDs keep it to a couple of dollars a month either way.

Going with a reef? Start with the 40-gallon reef cost breakdown. Staying fresh? Try the freshwater monthly cost calculator.

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