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Calculate a rectangular aquarium’s gross volume from its dimensions, estimate the water actually in the tank, then multiply that working volume by a chosen turnover rate to get a filter-flow target. The arithmetic gives you a planning figure—not a universal minimum: the right flow also depends on livestock, filter design, media, installation and circulation.
Calculate the tank’s gross volume
For a rectangular aquarium measured in inches, multiply length by width by height, then divide by 231. The result is the tank’s approximate capacity in US gallons. For measurements in centimeters, multiply the three dimensions to get cubic centimeters, then divide by 1,000 to get liters.
- US gallons: length (in) × width (in) × height (in) ÷ 231
- Liters: length (cm) × width (cm) × height (cm) ÷ 1,000
- Conversion: 1 US gallon = 3.785 liters = 231 cubic inches. ReefRig’s volume method uses this conversion.
Keep all three measurements in the same unit. This calculation assumes a rectangular box; it does not account for curved panels or other non-rectangular shapes.
Example: a 48 × 18 × 20-inch aquarium
48 × 18 × 20 ÷ 231 = about 75 US gallons of gross geometric volume. ReefRig gives this same example as approximately 283 liters. Its example estimates roughly 64 gallons of water after allowing for the fill line and rock and sand displacement. That reduction is an approximate, setup-specific estimate, not a fixed adjustment for every aquarium.
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- CARTRIDGE REFILLS: Change cartridges monthly; uses large-sized Tetra Whisper Bio-Bag Filter Cartridges (refills sold separately).
Estimate the working water volume
Gross volume describes the outside dimensions as a full rectangular box. The water in an operating aquarium is usually less: the tank may not be filled to its rim, and substrate, rock, decorations and equipment displace water. For a practical filter-flow estimate, use the best estimate of the water volume actually in service rather than assuming the geometric capacity is all water.
ReefRig suggests subtracting about 15% to estimate effective water volume. Treat that as a rough method from that publisher, not a universal rule; displacement depends on the fill level and the particular aquascape. If precision matters, calculate with internal dimensions and the actual water depth, and account for the setup’s displacement as closely as practical. ReefRig describes sizing flow and dosing against its estimated effective volume in its volume-calculation guidance.
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- QUIET AQUARIUM FILTER: The Tetra Whisper IQ Power Filter with Stay Clean technology contains a sound shield for quiet filtration less than 40 dB.
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- CAPACITY: The Tetra Whisper IQ 45 Power Filter works in tanks up to 45 gallons (215 GPH).
- STAY CLEAN TECHNOLOGY: By maintaining healthy pH levels, creates easier maintenance for healthier fish.
- CARTRIDGE REFILLS: Change cartridges monthly; uses large-sized Tetra Whisper Bio-Bag Filter Cartridges (refills sold separately).
Calculate turnover and target flow
Turnover rate is filter flow per hour divided by aquarium water volume in matching units. It expresses how many tank volumes the stated flow would move in an hour; it does not say how many times every drop is filtered.
- Turnover per hour: flow rate (L/h or GPH) ÷ water volume (L or US gallons)
- Target flow: working water volume × chosen turnover per hour
For example, if your working volume is 60 US gallons and you choose 5 turnovers per hour as a planning assumption, the calculation is 60 × 5 = 300 GPH. This tells you the flow associated with that chosen assumption; it does not establish that 5 turnovers per hour is right for your fish or filter setup. AquariumCalc likewise presents turnover as a sizing range to consider alongside bioload, filter design and species needs, rather than a universal minimum: AquariumCalc’s flow-rate guidance.
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Choose a filter by more than its GPH
A filter’s advertised flow rate is only one part of the comparison. Filter types—including hang-on-back, canister, internal, sponge and sump systems—differ in media capacity and the path water takes through the filter. Raw flow alone does not measure biological or mechanical filtration capacity.
- Check the exact model’s rating. Compare the manufacturer’s stated aquarium-size range and flow recommendation for that model; do not assume that ratings transfer between filters.
- Account for installed conditions. Media, plumbing, lift or head height, and buildup can affect output. The manufacturer’s rating may not describe the flow of your installed, maintained system.
- Match current to livestock. Fish and other inhabitants differ in their tolerance for strong current. A setup may need effective filtration without forcing animals to live in excessive flow.
- Look at circulation across the tank. Check whether water reaches areas where debris collects; a headline turnover figure alone does not guarantee even circulation.
- Use compatible media. If replacing filter media, verify that it fits and is suitable for your specific filter model.
The manufacturer’s manual for two EcoSystem Aquarium ProSeries models illustrates why model-specific advice matters. These are that manual’s suggestions, not general rules for other filters:
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- EASILY ADJUSTABLE: Adjustable clip allows for a variety of setups to easily adjust to high or low water levels.
| Model | Manufacturer-listed aquarium size | Suggested current flow |
|---|---|---|
| Pro 2410 | 40–65 gallons | 600–800 GPH |
| Pro 3012 | 75–100 gallons | 800–1,000 GPH |
The manual’s publication year is not established. Its figures apply to the named models and should not be treated as a universal turnover standard. See the EcoSystem Aquarium ProSeries manual for those model-specific recommendations.
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Use the calculator result as a shopping filter
- Measure the aquarium and calculate gross volume in gallons or liters.
- Estimate the working water volume, allowing for the fill level and displacement from substrate, rock and equipment.
- Select a turnover assumption appropriate to the species and system; the calculation cannot select that biological target for you.
- Multiply working volume by the chosen turnover to get a flow target in matching units.
- Compare aquarium filter models by their exact manufacturer ratings, media capacity, installation requirements and suitability for the livestock—not by GPH alone.
- After installation, assess current and circulation in the actual tank and maintain the filter as its manufacturer directs.
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