How much does a freshwater aquarium cost per month?

Less than most people guess, and the heater is usually the biggest line. The example below is a 20-gallon tropical tank with a 100 W heater, a hang-on-back filter and an LED light. Every value is editable, so swap in your own. Your inputs stay on this device.

Freshwater cost calculator

Electricity

Electricity rate ($ per kWh) Heater wattage (W) Heater duty cycle (% of the day heating) Filter wattage (W, runs 24 h) Light wattage (W) Light hours per day

Water and supplies

Water change, gallons per week Water + sewer rate, $ per 1,000 gallons Water conditioner bottle price ($) Bottle size (mL, dosed 5 mL per 50 gal) Test strips, price per pack ($) Strips per pack Tests per week Carbon inserts, price per pack ($) Inserts per pack Weeks between carbon changes (0 if you don't use carbon) Food, $ per month

Monthly

Yearly

Electricity only

Formulas: kWh/month = watts × hours/day × duty × 30.44 ÷ 1000. Monthly water gallons = gallons per week × 52 ÷ 12. Conditioner per month = gallons × 5 mL ÷ 50 × bottle price ÷ bottle mL. Food is left at $0 because there's no consistent public price data for it; work yours out with the food cost calculator and enter it here.

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. The 100 W heater is Fluval's M100, rated for tanks up to 30 gallons (Fluval), and the 30% duty cycle sits inside the 20–40% range ReefKalk gives for heaters in a heated room (source). The filter is an AquaClear 30, listed at 7 W (Fluval). The light is a Fluval Aquasky 2.0 for 24–36 inch tanks, 18 W (Fluval). $16.83 per 1,000 gallons is EPA WaterSense's 2024 US average residential water plus wastewater rate (EPA). Seachem Prime is dosed at 5 mL per 50 US gallons of new water (Seachem). Prices checked October 17, 2026: Prime 500 mL $18.99 and API 5-in-1 test strips (100 count) $24.98 at Bulk Reef Supply, and an AquaClear 30 carbon 3-pack $7.99 at Uncle Bill's Pet Centers. The 4-week carbon interval and 5 gallons a week (25% of a 20-gallon tank) are example choices, not manufacturer rules. 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.

Ways to cut freshwater running cost

Start with the heater. It's usually the biggest line. A lid, a spot away from outside walls and drafts, and a warmer room all cut its duty cycle. The heater cost guide shows how to measure yours.
Ask whether you need carbon. Carbon removes odors, discoloration and some impurities, but many tanks run fine without it. Set the carbon interval to 0 to see what skipping it saves, and keep using it if you're removing medication or tannins.
Buy conditioner by the dose, not the bottle. A concentrated conditioner in a big bottle costs pennies per water change. Compare by price per mL at your dose.
Buy test strips in bigger packs. In the example, a 100-count pack works out to about 25¢ a test. Check the per-strip price before you buy a small box.
Thinking about saltwater? See how the same tank compares in saltwater vs freshwater running costs.
Trim the photoperiod only if your plants allow it. LED lights are cheap to run, so this saves little money, but a shorter photoperiod can also mean less algae to clean.

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