Aquarium Flow & Filtration Calculator
Five interconnected calculators in one tool: filter turnover rate adjusted for bioload and tank type, head pressure and return pump sizing for sump systems, dead zone visual planner for powerhead placement, biological filter media volume, and annual running cost in your currency and electricity rate. All outputs in LPH, L/min, GPH (US & UK), and litres.
How to Use This Calculator
Each tab is a standalone module. Use any or all depending on your setup. Results update instantly.
Turnover & Filtration
Enter tank volume, tank type, and stocking level. Get the recommended filtration turnover in all units, a bioload adjustment, filter type correction, and a redundancy warning if one filter failure would be critical.
Head Pressure & Return Pump
Build your plumbing layout: vertical rise, pipe diameter, elbows, valves, and inline equipment. The calculator shows your total dynamic head and how much flow a pump loses at that resistance — so you buy the right size.
Circulation & Dead Zone Planner
Set tank dimensions and powerhead positions. A live top-view diagram shows estimated dead zones and flow coverage. Get a recommended powerhead count and positioning guide for your specific tank shape.
Filter Media Volume
Enter your fish stock details to calculate the minimum biological filter media volume needed. Compare ceramic rings, sintered glass, K1 fluidised bed, and bio-balls side by side.
Power & Running Cost
Add all your pumps and equipment. Enter your electricity unit rate in your local currency. Get daily, monthly, and annual running cost — and see the saving from switching to DC controllable pumps.
Flow & Filtration Calculator
Select a module below. All values update instantly as you type.
Filter Turnover Rate Guide — Every Tank Type
The "turnover rate" is how many times per hour your filter processes the total tank volume. These figures are for filtration turnover only — in-tank circulation from powerheads and wavemakers is separate and additional, particularly for reef tanks.
| Tank type | Turnover (filtration) | Total circulation | CO₂ note | Key consideration |
|---|---|---|---|---|
| Nano (<40 L) | 6–10× | 8–12× | — | Small water volume crashes fast — err toward higher turnover. Gentle flow to avoid stressing small fish. |
| Community freshwater | 4–8× | 6–10× | — | The standard recommendation. Adjust based on stocking density and how quickly nitrates rise between water changes. |
| Cichlids / goldfish | 8–12× | 10–15× | — | Very high waste production. These species are often the cause of persistent ammonia issues in undersized filters. |
| Planted — low tech | 4–6× | 5–8× | Protect CO₂ | Plants consume nitrate and help with biological load. Avoid surface agitation — it strips CO₂ from the water. |
| Planted — high tech CO₂ | 6–10× | 8–12× | Critical | High turnover for nutrient distribution, but outflow must be directed below the surface or CO₂ injection is wasted. |
| Discus | 6–10× | 8–12× | — | Discus produce moderate waste but are sensitive to ammonia. Gentle but consistent flow. Daily water changes often needed. |
| Saltwater — fish only | 8–12× | 10–15× | — | Marine fish produce more waste than equivalent-size freshwater fish. Skimmer highly recommended alongside mechanical filtration. |
| FOWLR | 8–12× | 12–20× | — | Live rock provides significant biological filtration. Aim for strong random flow to keep detritus in suspension. |
| Reef — soft coral | 8–12× | 15–25× | — | Soft corals and mushrooms prefer gentle, variable flow. Wavemakers on pulse or random mode recommended. |
| Reef — LPS coral | 8–12× | 20–35× | — | Torch corals, hammers, and brains need moderate turbulent flow. Avoid strong direct flow onto polyps. |
| Reef — SPS coral | 10–15× | 40–60× | — | Acropora and Montipora require very strong, random, multi-directional flow. Gyre pumps or multiple wavemakers essential. |
The Three Types of Aquarium Filtration
A well-functioning aquarium filter does three distinct jobs simultaneously. Understanding each type helps you choose the right filter media and explains why no single media type is enough on its own.
Biological filtration
The most important type. Colonies of beneficial bacteria convert toxic ammonia (from fish waste) into nitrite, then into less harmful nitrate. These bacteria live on every surface in the aquarium — but the filter media provides the highest concentration because it receives the greatest water flow. Biological filtration cannot be rushed — it takes 4–8 weeks to establish in a new tank and is permanently damaged by bleach, chlorinated tap water, and many medications. Never clean biological media under tap water.
Mechanical filtration
Physical removal of solid particles — uneaten food, fish waste, and plant debris — by trapping them in sponge, filter floss, or fine pads. This is the fastest-clogging part of your filter and needs the most frequent maintenance. A clogged mechanical stage restricts flow to the biological media beneath it, making both types less effective. In most canister and sump filters, mechanical media goes first in the flow path so it intercepts debris before it reaches biological media.
Chemical filtration
Removes dissolved compounds that mechanical and biological filtration cannot: yellowing tannins, dissolved organics, certain medications, chlorine, and heavy metals. Activated carbon is the most common chemical medium. It has a finite capacity and needs replacing every 4–6 weeks — once saturated it stops working. Zeolite removes ammonia and is useful for cycling new tanks or emergency situations. Chemical filtration is optional in mature, stable aquariums but beneficial for display tanks where water clarity matters.
Why all three matter
Each type removes different things that the others cannot. Biological filtration keeps ammonia and nitrite at zero but does nothing to clear particles or yellowing. Mechanical keeps the water clear but doesn't remove dissolved ammonia. Chemical polishes the water and handles emergencies but is ineffective against particulate or ammonia produced faster than the media can absorb. A complete filter system layers all three in the correct order: mechanical → biological → chemical (optional).
Filter Types Compared — Which Is Right for Your Setup?
| Filter type | Best for | Effective turnover | Media capacity | Maintenance | Verdict |
|---|---|---|---|---|---|
| Canister filter | Most freshwater tanks, planted tanks | 80–90% of rated LPH | High — large media basket | Every 4–8 weeks | Most versatile |
| Hang-on-back (HOB) | Small–medium freshwater, nano | 85–95% of rated LPH | Medium — often too small | Every 2–4 weeks | Easy maintenance |
| Sump / wet-dry | Large freshwater, reef, FOWLR | 95–98% of rated LPH | Very high — entire sump | Flexible — sections independently | Most powerful |
| Fluidised bed (K1) | High-bioload, large systems | N/A — media stays in suspension | Very high per litre of media | Minimal — self-cleaning | Highest biological |
| Sponge filter | Breeding tanks, fry, shrimp | Low — 50–70% of rated | Low | Weekly squeeze in tank water | Budget / gentle |
| Under-gravel filter (UGF) | Low-tech, static planted | 60–75% of rated LPH | Medium — uses substrate | Difficult — disrupts substrate | Declining use |
| Internal filter | Small tanks, quarantine | 70–85% of rated LPH | Low | Weekly | Beginner / small |
Two-filter strategy: Running two filters at 60–70% of required flow each is almost always better than one filter at 100%. If one fails — which is a matter of when, not if — the second maintains biological filtration long enough to prevent a crash. Clean filters alternately, never on the same day, to avoid losing too much bacterial colony at once.
Head Pressure — Why Your Pump Delivers Less Than Its Rating
Every pump you buy has a maximum flow rating on the box — but that rating is measured with zero resistance, in a straight horizontal pipe at ground level. Once you add vertical height, elbows, and inline equipment, the real-world flow drops significantly. This is head pressure.
Vertical rise (static head)
The most impactful factor. Every centimetre of vertical height the pump must push water upward adds resistance. A sump-to-display return with 120 cm of vertical rise is the most common example. The pump must work against gravity continuously, and manufacturers' head charts show exactly how flow rate drops with increasing height. A pump rated at 2,000 LPH may deliver only 1,200 LPH at 120 cm of rise.
Friction losses from fittings
Every fitting the water passes through adds resistance by creating turbulence and changing flow direction. A 90° elbow is the biggest single contributor — each one effectively adds the resistance equivalent of roughly 30 cm of straight pipe. Ball valves, check valves (which restrict in both directions), T-junctions, and inline equipment all add further resistance. A system with many fittings can lose 20–30% of rated pump flow from friction alone.
Pipe diameter matters
Narrow pipe creates more friction per metre than wide pipe. Running a 2,000 LPH pump through 12 mm pipe creates significant turbulence and back-pressure — the same pump through 25 mm pipe runs quieter and loses less flow. The correct rule is to size pipe diameter to the pump's outlet size or one step larger, never smaller. Reducing pipe diameter after a T-split is acceptable if the total cross-section doesn't decrease.
Inline equipment
UV sterilisers, inline heaters, calcium reactors, and media reactors are all flow restrictions. Manufacturers quote a maximum flow rate for each — running a UV steriliser faster than its rated flow reduces contact time and makes it ineffective. For UV to work, water must pass through slowly enough to receive a lethal UV dose. Always check the rated flow range for inline equipment and factor the resulting restriction into your head pressure calculation.
Always buy a pump rated for at least 30–40% more flow than you need at zero head. Use the head pressure module above to calculate your total dynamic head, then choose a pump whose manufacturer's flow curve shows your required LPH at that head height — not at zero.
Dead Spots — What Causes Them and How to Fix Them
What is a dead spot?
A dead spot is any area in the aquarium that receives little to no water movement. Detritus and fish waste settle here rather than being swept into the filter. Over time, this creates a localised anaerobic zone where beneficial bacteria cannot survive and hydrogen sulphide — a highly toxic gas — can build up. Disturbing a long-standing dead spot can release a hydrogen sulphide pulse into the water column, causing sudden fish death.
Where dead spots form
Rear corners are the most common location — flow from a single filter return or powerhead on one wall never reaches the diagonally opposite corner. Behind rockwork and aquascaping, the bottom of tall tanks where flow velocity is lowest, and underneath overhanging decorations are all typical dead zone locations. In reef tanks, the underside of large rock structures and behind the back wall are prime detritus traps.
The two-pump solution
Placing one powerhead on each short end of a rectangular tank, angled slightly inward and toward the opposite end, creates a collision zone in the middle of the tank and pushes flow into all four corners. Offsetting the height of the two pumps — one in the upper third, one in the lower third — ensures vertical coverage. This is the most effective basic setup for community freshwater and reef tanks alike.
Gyre-style circulation
Gyre pumps create a wide, gentle laminar flow across the entire tank length rather than a concentrated jet. When placed on the back wall, a gyre creates a rolling circular current — water moves across the back, deflects off the front glass, and returns along the sides. This pattern naturally reaches corners and substrate without creating the sand storms or stress that a focused powerhead can cause. Gyre-style circulation is now the standard approach in high-end reef keeping.
DC vs AC Pumps — The Running Cost Case
AC pumps (traditional)
AC pumps run at a fixed speed determined by mains frequency (50 or 60 Hz depending on your country). They are robust, cheap to buy, and proven over decades. The main disadvantage is inefficiency — they typically convert 40–50% of the electricity they consume into useful water movement, with the rest becoming heat. AC pumps cannot be throttled without a separate flow valve, which reduces flow but doesn't reduce power consumption.
DC pumps (controllable)
DC pumps use brushless motor technology with electronic speed control. They are 30–50% more energy-efficient than equivalent AC pumps and can be throttled to any speed via a dial, controller app, or smart home integration. DC pumps typically cost 2–3× more than AC equivalents but often pay back the difference in electricity savings within 18–36 months of continuous running. In a large reef system with multiple pumps running 24 hours a day, DC pumps can save thousands of rupees (or tens of pounds / dollars) annually.
The break-even calculation is straightforward: divide the price premium of the DC pump by the annual electricity saving. In regions with high electricity rates (UK, EU, parts of India), payback periods are as short as 12–18 months. Use Module 5 above to calculate exactly how much your current AC pumps cost to run and how much a DC switch would save.
Protein Skimmer & UV Steriliser Sizing
Protein skimmer sizing
Protein skimmers remove dissolved organic compounds before they break down into ammonia — effectively reducing the load on your biological filter. They are essential for reef tanks and beneficial for heavily stocked marine systems. Skimmer manufacturers rate their products by tank volume, but these ratings are almost always optimistic. A skimmer rated for 300 litres on a lightly stocked FOWLR might be appropriate for 150–200 litres of a heavily stocked reef. Always size up. The dwell time of water in the skimmer body is the key factor — a larger contact chamber removes more organics than raw flow rate.
UV steriliser flow requirements
UV sterilisers work by exposing water to ultraviolet light as it passes through a quartz chamber. The critical variable is contact time — water must pass slowly enough to receive a lethal UV dose for the target organism. To kill free-floating algae and some bacteria: 20–30 litres per hour per watt of UV. To kill ich and other parasites: 5–10 litres per hour per watt. To kill fish tuberculosis bacteria: 1–3 litres per hour per watt. Running a UV steriliser too fast renders it completely ineffective against parasites even if it clears green water. The head pressure module accounts for UV steriliser restriction in the overall plumbing calculation.
Frequently Asked Questions
How many times should my filter turn over the tank per hour?
Why does my pump deliver less flow than the rating says?
What is the difference between filtration turnover and tank circulation?
How do I know if I have dead spots?
How much biological filter media do I need?
Should I run one large filter or two smaller ones?
Does surface agitation affect CO₂ in planted tanks?
What flow rate does a UV steriliser need?
How do I calculate my aquarium's annual running cost?
Is a higher turnover rate always better?
Complete Your Aquarium Planning Toolkit
Once you know your filtration flow requirements, use our Tank Volume Calculator to get the precise system volume — in litres, US gallons and UK gallons — accounting for glass thickness, substrate depth and decorations. Accurate volume is the foundation of every flow calculation.
Then use the Stocking Calculator to check whether your current fish community is generating the bioload level you've configured here — with live compatibility warnings and filtration-adjusted limits for 100+ species.
Use the Water Change Calculator to plan the maintenance schedule that keeps nitrate below the levels your filtration system produces.