What does a 40-gallon reef tank really cost per month?
The worked example below comes to about $34 a month before food and additives, and it's deliberately on the high side: pumps and light are set to their rated maximum draw. Electricity is about half the bill, and salt plus RODI water is most of the rest. Every value is editable, and your inputs stay on this device.
Other tank sizes: pick one above. The 40-gallon preset won't match the worked example below exactly. It uses two Prime 16HDs to cover the 36-inch length, a return pump sized by BRS's 5× rule (16 W) and a 10% weekly water change (4 gallons). Use "Restore the example values" in the preset box to get back to the worked example.
40-gallon reef cost calculator
Electricity
Water, salt and testing
Energy use:
Formulas: kWh/month = watts × hours/day × duty × 30.44 ÷ 1000. Salt = weekly water-change gallons × 52 ÷ 12 × price ÷ rated gallons. RODI = (water-change gallons + top-off gallons) × cost per gallon. Testing = tests per week × 52 ÷ 12 × kit price ÷ tests per kit. Food, additives and media are left at $0 because they vary too much to pre-fill honestly; enter your own.
The worked example, line by line
| Line | Example inputs | $/month |
|---|---|---|
| Heater | 150 W × 24 h × 30% duty = 32.9 kWh | $5.92 |
| Light | 59 W × 8 h = 14.4 kWh | $2.59 |
| Pumps & skimmer | (35 + 19 + 9) W × 24 h = 46.0 kWh | $8.28 |
| Salt | 21.7 gal a month × $89.99 ÷ 200 gal | $9.75 |
| RODI water | (21.7 + 15.2 top-off) gal × $0.175 | $6.46 |
| Testing | 4.3 alkalinity tests × $0.188 | $0.81 |
| Total | 93.3 kWh at $0.18, plus consumables | $33.81 |
That's about $406 a year. The pumps are at their maximum rated draw, so if you run the return pump and powerhead turned down, the electricity line will be lower: the same two pumps at their rated minimum (6 W and 8 W) would cut the pump line from $8.28 to $3.02.
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. Fluval's M150 is 150 W and rated for tanks up to 45 gallons (Fluval), and the 30% duty cycle sits inside the 20–40% range ReefKalk gives for heaters in a heated room. For the light, the AquaIllumination Prime 16HD lists 59 W at full power and a 24 × 24 inch spread (AI); a 36-inch-long tank may need two. For flow, 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), and the Reef Octopus Classic 110-INT skimmer pump draws 9 W on 60 Hz US power (Reef Octopus); we use the maximum ratings. BRS recommends a 10% weekly water change, which it puts at 5 gallons for a 40-gallon tank (BRS). BRS says a 40-gallon tank evaporates about half a gallon a day (BRS). 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 cost per gallon calculator (EPA water rate, BRS filter prices); at fewer gallons a month the fixed filter costs push it up a little, so run that calculator with your own volume. 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". Testing alkalinity once a week is an example choice, not a rule. Prices change, so enter what you pay.
Where the tank-size preset numbers come from (checked October 17, 2026)
- Tank sizes: the presets cover the most common US standard sizes, with standard footprints rounded to the inch (L × W × H): 5.5 gal 16 × 8 × 10, 10 gal 20 × 10 × 12, 15 gal 24 × 12 × 12, 20 Long 30 × 12 × 12, 20 High 24 × 12 × 16, 29 gal 30 × 12 × 18, 40 Breeder 36 × 18 × 16, 55 gal 48 × 13 × 21, 75 gal 48 × 18 × 21, 90 gal 48 × 18 × 24, 125 gal 72 × 18 × 22 and 180 gal 72 × 24 × 24. These match Aqueon's published dimensions within about an inch (Aqueon PDF, Aqueon size chart). "Most popular" marks our estimate of the most common sizes, not measured sales data. No maker lists a standard 300 or 500, so those are large-system estimates (96 × 24 × 30 in and 96 × 36 × 34 in), checked by volume: L × W × H ÷ 231 = gallons. Reef presets start at 20 gallons. We didn't find solid published equipment data for nano reefs under 20 gallons, so the 5.5, 10 and 15 are freshwater only.
- Heaters: Fluval M series ratings: M50 up to 15 gal, M100 up to 30, M150 up to 45, M200 up to 65, M300 up to 80 (Fluval). We use the smallest one that covers the tank. Above 80 gallons we split the volume across enough heaters that each covers its share. As a cross-check, Aqueon's chart lists 300 W for 65–90 gallons (Aqueon). The 30% duty cycle sits inside the 20–40% range ReefKalk gives for heaters in a heated room.
- Freshwater filters: Fluval AC20 (5–20 gal), AC30 (10–30), AC50 (20–50) and AC70 (40–70) are 7 W each, and the AC110 (60–110) is 14 W (Fluval AC). The Fluval FX2 is rated to 175 gal at 27 W, the FX4 to 250 gal at 30 W and the FX6 to 400 gal at 43 W (Fluval FX). We pick the smallest filter whose top rating is above the tank size.
- Freshwater lights: Fluval Aquasky 2.0: 12 W for 15–24 in tanks, 18 W for 24–36 in, 27 W for 36–48 in and 35 W for 48–60 in (Fluval). Tanks longer than 60 in get two equal fixtures.
- Reef lights: the AquaIllumination Prime 16HD draws 59 W at full power with a 24 × 24 in spread (AI). We count how many it takes to cover the tank's length and width. Larger fixtures exist; we use one model so the math stays easy to check.
- Return pumps: BRS's rule is a pump rated at least 5 to 10 times the tank's volume per hour (BRS). We pick the smallest Sicce Syncra Silent rated at 5 times or more, using its US 120 V specs: 0.5 is 185 gph at 8 W, 1.0 is 251 gph at 16 W, 1.5 is 357 gph at 23 W, 2.0 is 568 gph at 35 W, 3.0 is 714 gph at 48 W and 4.0 is 951 gph at 85 W (Sicce US). For 300 and 500 gallons we use the Syncra SDC 7.0 (7,000 L/h, about 1,849 gph, 20–65 W) and 9.0 (9,000 L/h, about 2,378 gph, 30–90 W) (Sicce SDC).
- Powerheads: BRS says most reef tanks do well with 20 to 30 times the tank volume per hour of internal flow, and that most tanks under 100 gallons do well with two powerheads, while some small tanks can start with one (BRS). EcoTech's VorTech MP10mQD is 2,500 gph at 8–19 W for 2.5–50+ gal tanks, the MP40mQD is 5,000 gph at 9–37 W for 50–500+ gal, and the MP60mQD is 9,000 gph at 10–60 W for 120–1,000+ gal (EcoTech). We use one MP10 up to 20 gallons and at least two pumps above that, enough to reach 20 times the volume at full output.
- Skimmers: Reef Octopus Classic 110-INT is 9 W and rated to 132 gal, the 150-INT is 18 W and rated to 211 gal, and the 200-INT is 35 W and rated to 317 gal (Reef Octopus).
- Water changes: 10% a week. BRS recommends that for reefs (BRS), and Aqueon says to change 10% every week or 25% every two weeks for freshwater (Aqueon).
- Top-off: BRS says a 40-gallon tank evaporates about half a gallon a day (BRS). We scale that by each tank's water surface (length × width), so top-off figures are estimates.
- Supplies: the same dated, sourced prices as the rest of the site. Instant Ocean salt is $89.99 per 200 gallons, the Salifert alkalinity kit $18.79 for about 100 tests, API 5-in-1 strips $24.98 per 100, and Seachem Prime $18.99 per 500 mL at 5 mL per 50 gallons. AquaClear 30 carbon is $7.99 for a 3-pack, changed every 4 weeks; carbon for other filters isn't priced, so it starts at $0. RODI water is $0.175 a gallon from our RODI calculator. In the main calculator that's a $17.99 resin refill lasting about 938 gallons (3,750 ppm-gallons at 200 ppm tap water and 98% rejection), plus $52.96 a year for prefilters and the membrane. Food isn't pre-filled.
Pumps and lights are set to their maximum rated draw, so the presets are a high-end estimate. Most DC pumps and LEDs run below their maximum.
Where a 40-gallon reef's money goes, and how to cut it
- Pumps run all day, every day. Pump watts are small but add up because they never stop. If your DC pumps run below full speed, enter what they actually draw. A plug-in energy meter shows the real number.
- Salt is a per-gallon decision. Compare by cost per gallon mixed, not bucket price. Our salt comparison has the major brands side by side.
- RODI water is cheap per gallon, but you use a lot of it. Top-off alone is about 15 gallons a month in this example. A lid or cover cuts evaporation, which lowers both RODI use and heater run time.
- Heater duty cycle swings with the seasons. The heater cost guide shows how to measure yours.
- LEDs are cheap to run. At 59 W for 8 hours, the light is under $3 a month here. If you're weighing T5s, see LED vs T5 lighting cost.