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How much does a 5.5-gallon aquarium cost per month?
A typical 5.5-gallon freshwater tank costs about $4.32 a month to run ($51.83 a year), before food. We don't give a full reef figure for this size; see why below.
Try it with your own numbers
Open the calculator: 5.5-gallon freshwater
Each button opens the running-cost calculator with this size's typical setup already filled in, so you can swap in your own equipment and electricity rate. If you've changed the calculator's electricity rate or food before, it keeps your numbers for those.
What these numbers assume
- Electricity: 17¢ per kWh, the calculator's default, which ReefKalk cites as about the US average. Every 1¢ per kWh adds about $0.19 a month to the freshwater tank. At the US average residential price of 18.31¢ for July 2026 (EIA Electric Power Monthly, Table 5.6.A), the totals would be $4.57 freshwater. Colorado's average was 17.00¢, the same as our default.
- Heater: Fluval M50 (50 W), the smallest Fluval M heater rated for 5.5 gallons, heating 30% of the time. That share sits inside the 20–40% range ReefKalk gives for a heated room. A cold room or a winter month can push it higher.
- Freshwater gear: a Fluval AC20 filter (7 W, runs all day) and a Fluval Aquasky 2.0 for 15–24 in tanks (12 W) on 8 hours a day.
- Water changes: 10% a week, about 0.55 gallons. Top-off for evaporation is about 0.1 gallons a day, scaled from BRS's half a gallon a day for a 40-gallon tank by water surface area, so it's an estimate.
- Supplies: Seachem Prime conditioner and one API 5-in-1 test strip a week for freshwater. Prices were checked October 15–17, 2026.
Full sources are in where the numbers come from below.
The math, line by line
5.5-gallon freshwater
| Line | What we assume | $/month |
|---|---|---|
| Heater | Fluval M50, 50 W × 24 h × 30% of the time | $1.86 |
| Light | Fluval Aquasky 2.0, 15–24 in fixture, 12 W × 8 h | $0.50 |
| Filter | Fluval AC20, 7 W × 24 h | $0.87 |
| Testing | 1 test strip a week ($24.98 per 100) | $1.08 |
| Water conditioner | 5 mL per 50 gal of new water, $18.99 per 500 mL | $0.01 |
| Filter carbon | optional; not priced for this filter, so $0 | $0.00 |
| Total | 19.0 kWh at 17¢, plus supplies | $4.32 |
Formulas (the same ones the calculator uses): kWh a month = watts × hours a day × share of time on × 30.44 ÷ 1,000. Monthly water-change gallons = gallons a week × 52 ÷ 12, and top-off = gallons a day × 30.44. The freshwater total doesn't count tap water itself. At EPA WaterSense's 2024 US average water and sewer rate of $16.83 per 1,000 gallons, this tank's water changes add about $0.04 a month. Food is left at $0 on both sides because there's no consistent public price data for it. Work yours out with the food cost calculator.
What about a 5.5-gallon reef?
We don't give a full monthly figure for a 5.5-gallon reef. We didn't find solid published equipment data for nano reefs under 20 gallons, so the calculator's reef presets start at the 20 Long. Here's what the supplies alone would cost, using the same rules as the bigger tanks:
- Salt: a 10% weekly change is about 2.4 gallons a month. At Instant Ocean's $89.99 per 200 gallons, that's $1.07 a month.
- RODI water: about 5.4 gallons a month for changes and top-off. At the $0.175 a gallon from our RODI calculator's example, that's $0.95. That example assumes 50 gallons a month, though. A home RODI system making this little water spreads its fixed filter costs over fewer gallons, so your real cost per gallon would be higher.
- Testing: one Salifert alkalinity test a week is $0.81 a month.
That's about $2.83 a month in supplies, before electricity for the heater, light and pumps. You can enter your own nano equipment in the running-cost calculator.
Where to cut costs on a 5.5-gallon tank
- Start with the heater. It's $1.86 a month here, 43% of the freshwater total. Every 10 points of heater run time (say, 30% down to 20%) is worth about $0.62 a month. A lid, a spot away from outside walls and drafts, and a warmer room all cut it. The heater cost guide shows how to measure yours.
- Buy conditioner and test strips by the dose. A concentrated conditioner costs pennies per water change, and a 100-count strip pack works out to about 25¢ a test.
5.5-gallon tank specs
| Outside dimensions (L × W × H) | 16 3/16 × 8 3/8 × 10 1/2 in (Aqueon); about 16 × 8 × 10 in rounded |
|---|---|
| Empty glass tank | 7 lb |
| Full, as listed by Aqueon | 62 lb, before substrate, rock and décor |
| Water alone | about 46 lb freshwater, 47–47 lb saltwater |
| Weekly water change (10%) | about 0.55 gallons |
| Evaporation (estimate) | about 0.1 gallons a day |
| Heater | Fluval M50 (50 W, rated to 15 gal). Aqueon's chart suggests 50, 50 and 50 W for a 5 gallon tank to raise it 5, 10 or 15 °F above the room. |
Dimensions and weights are from Aqueon's critical dimensions chart. Aqueon notes that filled weights vary with substrate and décor. Other brands' tanks of the same size differ by a fraction of an inch. Water weight is the tank's nominal 5.5 gallons × 8.34 lb per gallon (USGS). Seawater is about 1.02–1.03 times as dense as fresh water (WHOI). The heater chart is from Aqueon's size and heater chart. Make sure your stand and floor are rated for the full weight.
FAQ
How much does a 5.5-gallon aquarium cost per month?
About $4.32 a month ($51.83 a year) for a typical 5.5-gallon freshwater tank, before food, at 17¢ per kWh. Electricity is 75% of that, mostly the heater.
How much does a 5.5-gallon reef tank cost per month?
We don't publish a full figure because we found no solid published equipment data for nano reefs. The supplies alone (salt, RODI water and weekly alkalinity tests) come to about $2.83 a month before electricity.
How much does a 5.5-gallon aquarium weigh?
Aqueon lists its 5.5-gallon tank at 7 lb empty and 62 lb full, before substrate, rock and décor. The water alone is about 46 lb fresh or 47–47 lb salt.
What size heater does a 5.5-gallon tank need?
Fluval's M50 (50 W) is the smallest in its M series rated for 5.5 gallons. Aqueon's chart suggests 50, 50 and 50 W to hold a 5 gallon tank 5, 10 or 15 °F above a 68–72 °F room.
How much water should I change in a 5.5-gallon tank each week?
About 0.55 gallons, which is 10% of the tank. BRS recommends 10% a week for reefs, and Aqueon suggests 10% a week or 25% every two weeks for freshwater.
Other tank sizes
- 5.5-gallon tank: about $4.32 a month freshwater (freshwater only) (this page)
- 10-gallon tank: about $4.33 a month freshwater (freshwater only)
- 20-gallon long tank: about $9.34 a month freshwater, $22.53 reef
- 55-gallon tank: about $10.61 a month freshwater, $37.71 reef
- 75-gallon tank: about $15.23 a month freshwater, $51.58 reef
Other sizes: the 40-gallon reef breakdown, or pick any of 14 sizes in the calculator. All sizes side by side →
Where the numbers come from: every equipment wattage, water-change rule and supply price comes from the calculator's tank-size presets (sources listed below). The 17¢ default rate is from ReefKalk, and the 18.31¢ US and 17.00¢ Colorado averages for July 2026 are from the EIA Electric Power Monthly, Table 5.6.A. Water weight is from USGS and seawater density from WHOI. Tank dimensions, weights and the heater chart are from Aqueon (chart). Prices change, so enter what you pay in the calculator.
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.