Every aquarium calculator, in one place.

Step 1: Choose the right tank size

Beginners are often steered toward small tanks because they seem easier. In practice the opposite is true. Water volume is a buffer: in a large tank, a missed feeding, a dead snail behind a rock or a heater that sticks for an hour changes the water chemistry slowly, giving you time to notice and react. In a 5-gallon (19-liter) tank the same mistake can push ammonia or temperature to dangerous levels within a day. For a first tank, something in the 20–40 gallon (75–150 liter) range is far more forgiving than a nano, while still fitting on ordinary furniture-grade stands.

Shape matters as much as volume. Fish live in the horizontal plane, and gas exchange happens at the surface, so a long, shallow tank supports more life than a tall one of the same capacity. A standard 20-gallon long (30 × 12 × 12 inches) has a much larger footprint than a 20-gallon high (24 × 12 × 16 inches), which is why it is the better choice for active schooling fish. Tall tanks and hexagons look striking but give fish less swimming length and less surface area for oxygen to enter the water. Our stocking calculator accounts for this by weighting capacity by footprint, not just liters.

Finally, decide what will live in the tank before you buy the glass, not after. Many popular fish have a minimum length requirement that has nothing to do with gallons: a fish that grows to 6 inches needs room to turn around and swim, whatever the volume says. Check the adult size and minimum tank size of every species on your wish list in the species directory, and let the most demanding one set the tank dimensions.

Step 2: Work out the real water volume

The volume printed on the box is the outside dimensions of the glass multiplied together, and it is almost always more than the water the tank really holds. Three things reduce it. First, the glass itself has thickness, so the inside length, width and height are each slightly smaller than the outside. Second, nobody fills a tank to the very top edge: the water line usually sits 2–4 cm (about 1–1.5 inches) below the rim, under the frame or lid. Third, substrate, rock, driftwood and equipment all take up space that would otherwise be water.

Here is how much that adds up to on a typical 240-liter tank measuring 120 × 40 × 50 cm on the outside with 10 mm glass. The inside dimensions are about 118 × 38 × 49 cm, which is roughly 220 liters. Filling to 3 cm below the rim brings it to about 206 liters. A 5 cm sand bed removes another 22 liters, and a modest hardscape displacing 5% takes off around 9 more. The real water volume is close to 175 liters, almost 30% less than the label.

That difference matters because nearly every other number in fishkeeping is a ratio of water volume. Medication and fertilizer doses, water conditioner, heater wattage, filter turnover and stocking capacity are all calculated per liter or per gallon. Dosing a 175-liter tank as though it held 240 liters means overdosing by more than a third, which is harmless with some products and dangerous with others, particularly copper-based treatments and some invertebrate-sensitive medications.

The volume calculator at the top of this page handles all of this. Enter your dimensions in inches or centimeters, choose your tank shape, then add your substrate depth and a décor displacement percentage to get the net volume. Use that net figure everywhere else in this guide. If you only know the volume you want and the space you have, reverse mode will solve for the missing dimension.

Step 3: Plan for weight and placement

Water is heavy: 1 liter weighs about 1 kilogram, and a US gallon weighs about 8.34 pounds. Once you add glass, substrate, rock and equipment, a filled tank weighs considerably more than the water alone. A standard 55-gallon setup is well over 600 pounds (about 280 kg) when running, concentrated on a footprint of roughly 4 feet by 1 foot. Saltwater adds about 2.5% to the water weight, and live rock is denser still.

Use a stand designed for aquariums, not a bookcase or a dresser, and check that it sits perfectly level before filling. An uneven stand puts twisting stress on the glass seams, and the tank should be fully supported around its frame with no gaps. On upper floors, position larger tanks against a load-bearing wall and perpendicular to the floor joists so the weight is spread across several of them. If you are planning anything above about 75 gallons (280 liters) on a suspended floor, or you are unsure how your floor is built, ask a builder or structural engineer before you fill it.

Location affects more than structure. Direct sunlight drives algae and swings temperature, radiators and air vents cause the same problem, and a spot next to a door that slams will stress fish. Leave room behind the tank for filter hoses and cables, put the tank near a grounded outlet with a drip loop on every cord, and think about how you will carry buckets to and from it every week. The tank weight calculator estimates empty and fully loaded weight from your dimensions and glass thickness so you can check the stand rating before you buy.

Step 4: Size the filter by turnover

A filter does two jobs. Mechanically, it traps waste particles so the water stays clear. Biologically, and much more importantly, it provides surface area for the bacteria that convert toxic ammonia from fish waste into nitrite and then into the far less harmful nitrate. A tank can look crystal clear and still poison its fish if the biological filter is too small or not yet established, so the goal is enough flow and enough media for the bioload, not just clarity.

Filters are compared by turnover: how many times per hour the full water volume passes through. For most freshwater community tanks, 4× per hour is a sensible minimum, around 6× is a good target, and heavily stocked tanks or messy fish such as goldfish and large cichlids benefit from 8–10×. For a 150-liter (40-gallon) tank, that means a filter delivering roughly 600–900 liters per hour (160–240 GPH), with more for a heavy load. Planted tanks with gentle fish such as bettas or small rasboras can run toward the lower end, because strong current tires weak swimmers.

Be skeptical of the flow rate printed on the box. Manufacturers measure it with no media, no hoses and no height for the pump to lift water against. Once a canister is packed with media, connected with hoses and running under a cabinet, its real flow is often a third to a half lower than the rating, and it drops further as the media clogs between cleanings. That is why it is common advice to buy a filter rated for a tank larger than yours.

The type of filter matters less than the capacity. Hang-on-back filters are simple and cheap, canister filters hold much more biological media and run quietly, and air-driven sponge filters are gentle, nearly indestructible and ideal for fry, shrimp and hospital tanks. Many keepers run two filters on larger tanks so one keeps the biology going while the other is cleaned. When you clean media, rinse it in old tank water, never tap water, because chlorine kills the bacteria you spent weeks growing. The filter calculator compares your filter's flow with your net volume and tells you whether turnover is low, healthy or excessive.

Step 5: Choose a heater that matches your room

Most tropical community fish are kept between 24 and 27 °C (75–80 °F). The heater's job is to hold that temperature against the room around it, which means the wattage you need depends far more on how much colder the room gets than on the tank alone. A tank in a centrally heated living room that rarely drops below 21 °C needs very little help. The same tank in an unheated basement or a room that cools to 15 °C overnight needs roughly twice the wattage.

A transparent way to size it: allow about 0.2 W per liter of net volume for every degree Celsius between your target and the coldest the room gets. A 100-liter tank kept 5 °C above the room needs about 100 W; kept 10 °C above a cold room, about 200 W. This lines up with the traditional "1 watt per liter" rule for a typical heated home and explains why that rule fails in cold rooms. Always round up to the next real heater size, because a heater running flat out with no headroom will fall behind on the coldest night.

On tanks above roughly 200 liters (55 gallons), split the wattage across two smaller heaters at opposite ends of the tank. Heat spreads more evenly, and if one heater fails, whether stuck off or stuck on, the other limits the damage. Heaters failing in the "on" position are one of the most common causes of mass fish loss, so many keepers plug them into a separate temperature controller as a safety cut-off. Keep a cheap independent thermometer in the tank too; heater dials are often several degrees off. Coldwater species such as goldfish, white cloud minnows and many hillstream loaches do not need a heater at all in a normal indoor room. The heater calculator works out the wattage from your volume, target temperature and room temperature, and recommends a standard heater size.

Step 6: Pick and measure your substrate

Substrate is not just decoration. It hosts beneficial bacteria, anchors plants, affects water chemistry and determines which fish will be comfortable. Bottom-dwellers with sensitive barbels, such as corydoras catfish and many loaches, should be kept on fine, smooth sand rather than sharp gravel. Rooted plants grow best in nutrient-rich aquasoil or in gravel with root tabs. Crushed coral and aragonite slowly raise pH and hardness, which suits African cichlids and marine tanks but not soft-water species from the Amazon.

Depth depends on what the substrate is for. A sand bed for a fish-only tank works well at 2.5–5 cm (1–2 inches); deeper sand can trap pockets of stagnant, oxygen-free water unless it is stirred by snails or burrowing fish. Gravel for a community tank is typically 5 cm (2 inches). Planted tanks want 5–8 cm (2–3 inches), often sloped deeper at the back to give room for larger root systems and to create depth in the layout.

To buy the right amount, multiply the base area by the depth to get volume, then by the material's density to get weight. Aquarium gravel weighs about 1.5 kg per liter, sand about 1.6 kg per liter, and lightweight aquasoil only around 0.8 kg per liter, which is why a bag of soil covers much more area than a bag of sand of the same weight. A 120 × 40 cm tank with a 5 cm sand bed needs 24 liters of sand, about 38 kg. Rinse sand and gravel thoroughly before adding them, but do not rinse aquasoil, which breaks down if handled roughly. The substrate calculator returns the weight and volume to buy for any base size and depth, and how much water the bed will displace.

Step 7: Cycle the tank before adding fish

The single most important step in setting up a new aquarium is the one with no equipment to buy: growing the bacteria that make the water safe. Fish constantly release ammonia through their gills and waste. In an established tank, one group of bacteria converts that ammonia into nitrite, and a second group converts nitrite into nitrate. Both ammonia and nitrite are toxic at very low levels. Nitrate is far less harmful and is removed by water changes and plant growth. This chain is called the nitrogen cycle, and a brand-new tank does not have it yet.

Adding a full stock of fish to an uncycled tank is the classic cause of "new tank syndrome": the water looks perfect, but ammonia climbs over the first week, fish become lethargic and gasp at the surface, and losses begin. The fix is to cycle the tank first. In a fishless cycle you add a source of ammonia, such as pure liquid ammonia or a small amount of fish food, and wait while the bacteria colonies grow. It typically takes two to six weeks. The tank is ready when it can process a dose of about 2 ppm ammonia down to zero ammonia and zero nitrite within 24 hours.

You can speed things up by seeding the new filter with media or a sponge from an established, healthy tank, or with a bottled bacteria product, but you should still confirm the result with tests rather than assume it. A liquid test kit for ammonia, nitrite, nitrate and pH is one of the best investments a new fishkeeper can make; test strips are convenient but less precise. Live plants help as well, because they consume ammonia and nitrate directly.

Once the tank is cycled, add fish gradually rather than all at once. The bacteria population matches the waste it is fed, so a sudden jump in bioload can overwhelm it until the colonies catch up. Adding a few fish at a time, a week or two apart, and testing ammonia and nitrite after each addition keeps the system in balance. If you can, quarantine new arrivals in a separate small tank for two to four weeks first, which stops a single sick fish from infecting an entire established community.

Step 8: Stock by bioload, not by inches

The best-known stocking guideline is "one inch of fish per gallon". It is simple, and it is wrong often enough to cause real problems. It treats a slender 1-inch neon tetra and a deep-bodied 1-inch juvenile goldfish as identical, even though the goldfish will grow to many times the length and produce far more waste. It ignores the difference between the fish's size today and its adult size, and it says nothing about behavior, swimming space or whether the species can live together at all.

A better approach is to think in bioload: the amount of waste each species produces relative to its adult body mass and feeding habits. A neon tetra has a tiny bioload; a common pleco or an oscar has an enormous one. Total the bioload of everything you plan to keep, compare it with your tank's net volume and footprint, and make sure your filtration and water-change schedule can keep up. This is how our stocking calculator works: each species carries a bioload factor, and the tank's capacity is based on net liters and adjusted for footprint, so a long, shallow tank scores higher than a tall one of the same volume.

Numbers are only half of stocking. Many popular fish are schooling or shoaling species that are stressed, pale and more prone to disease when kept in twos or threes; tetras, rasboras, corydoras and many barbs should be kept in groups of at least six, and they look and behave much better in larger groups. Some species are fin-nippers that will harass long-finned tank mates such as bettas and fancy guppies. Others are territorial, need different water hardness or temperature, or will eat anything that fits in their mouths. Every species in your plan must share an overlapping range of temperature and pH.

Aim for a stocking level that leaves headroom rather than one that sits at the limit. A lightly to moderately stocked tank is more stable, needs less aggressive maintenance, and gives you room to absorb fish that grow larger than expected or the occasional surprise batch of fry. The stocking calculator flags overstocking, under-filtration, schooling numbers that are too small, temperament clashes, parameter mismatches and tanks that are too small for a species, with an explanation for each warning so you understand why it matters.

Step 9: Set a water-change routine that works

Even with a perfect biological filter, nitrate builds up steadily, along with dissolved organic compounds and hormones, while minerals that buffer pH are slowly used up. Regular partial water changes remove the waste and restore the minerals. They are the most effective piece of maintenance you can do, and no additive or gadget replaces them.

The useful math here is dilution. Each change removes the same fraction of whatever is dissolved in the water. Replace 25% and nitrate drops by a quarter; replace 50% and it halves. To bring nitrate from 80 ppm down to 20 ppm takes two 50% changes, or five 25% changes. More importantly, with a steady routine, nitrate settles at a predictable level: it peaks at roughly the amount your tank produces per week divided by the fraction you change each week. A tank producing 5 ppm of nitrate per week stays around 20 ppm with weekly 25% changes, but climbs toward 50 ppm with weekly 10% changes.

For most community tanks, 20–30% once a week is a sound default. Lightly stocked planted tanks can often stretch to every two weeks, while heavily stocked tanks, goldfish and large cichlids may need 40–50% weekly. Several smaller changes are gentler than one huge one, because a large change of water with a different temperature, pH or hardness can shock fish. This is especially true for tanks that have been neglected: if nitrate is very high and pH has drifted, bring it back with a series of moderate changes over several days rather than a single 90% change.

Always treat new tap water with a dechlorinator that neutralizes both chlorine and chloramine, and match the temperature to within a degree or two of the tank. Vacuum the substrate during changes to remove trapped waste, but in planted tanks only skim the surface so roots are not disturbed. The water-change calculator tells you how many liters or gallons a given percentage is, how far a series of changes will dilute nitrate, and how many changes it takes to reach your target level.

Putting it together: a 20-gallon long, start to finish

To show how the steps connect, here is a complete plan for one of the most popular community tanks: a 20-gallon long measuring 30 × 12 × 12 inches. Its gross volume is 4,320 cubic inches, which is about 70.8 liters or 18.7 US gallons. That is already slightly under the "20 gallons" on the label before anything goes into it.

With a 2-inch sand bed, the substrate occupies 30 × 12 × 2 inches, about 11.8 liters. At 1.6 kg per liter, that is roughly 19 kg (42 lb) of sand to buy, and it brings the net water volume down to about 59 liters (15.6 US gallons) before décor. Filled and running with glass, sand, rock and equipment, the whole tank weighs in the region of 100 kg (about 220 lb), which any purpose-built 20-gallon stand handles easily but which is too much for many shelves and side tables.

For filtration, 59 liters at a target of 6× turnover calls for about 350 liters per hour (around 95 GPH) of real flow. Allowing for the drop between a box rating and real-world performance, a filter rated for 30–40 gallon tanks is a sensible choice, or a hang-on-back filter plus a sponge filter for redundancy. To hold 25 °C (77 °F) in a room that drops to 20 °C (68 °F), the heater needs about 59 × 5 × 0.2 ≈ 59 W, which rounds up to a standard 75 W heater.

For stocking, the long footprint suits a peaceful community: for example, a school of ten to twelve small tetras or rasboras in the middle of the water, a group of six pygmy or panda corydoras on the sand, and a few nerite snails for algae. That combination keeps the bioload comfortable for the filter, gives each schooling species a proper group, and avoids fin-nippers and territorial fish. It would be cycled for three to six weeks first, then stocked in two or three stages.

Maintenance would start with a weekly 25% water change of roughly 15 liters (4 US gallons), adjusted once nitrate readings show how quickly the tank produces waste. Every number in this example came from one of the calculators on this site, and you can recreate it for your own tank in a few minutes, then share the results by link with a friend, a forum or your local fish store.

Eight common mistakes and how to avoid them

Most of these come up again and again on fishkeeping forums. Each one is easy to prevent with a little planning.

• Adding fish on day one. An uncycled tank cannot process ammonia. Cycle first, test, then stock gradually.
• Trusting the label volume. Real water volume is often 15–30% lower than the box. Dose medications and size equipment from the net volume.
• Buying fish for their current size. Many store fish are juveniles. Plecos, goldfish, many cichlids and some sharks grow several times larger. Plan for adult size and adult behavior.
• Keeping schooling fish in pairs. Tetras, rasboras and corydoras need groups of six or more to feel secure and show natural behavior.
• Cleaning filter media under the tap. Chlorine kills the bacteria that keep the tank safe. Rinse media gently in removed tank water, and never replace all of it at once.
• Overfeeding. Uneaten food rots and becomes ammonia. Feed only what disappears in a minute or two, once or twice a day.
• Relying on one heater with no backup. A heater stuck on can cook a tank overnight. Use a thermometer you trust, and on bigger tanks split the wattage or add a controller.
• Skipping water changes because the water looks clear. Nitrate and dissolved waste are invisible. Test regularly and keep to a schedule.

Quick-reference numbers

Rules of thumb worth keeping in mind. They are starting points, not substitutes for testing your own water and checking your own species.

• Water weight: about 1 kg per liter, or 8.34 lb per US gallon. Saltwater is about 2.5% heavier.
• Gallons and liters: 1 US gallon = 3.785 liters, 1 UK gallon = 4.546 liters, and 231 cubic inches = 1 US gallon.
• Filter turnover: 4× the net volume per hour as a minimum, about 6× as a target, and 8–10× for heavy bioloads.
• Heater wattage: about 0.2 W per liter for each °C above the coldest room temperature, rounded up to the next heater size.
• Tropical temperature: 24–27 °C (75–80 °F) for most community fish.
• Substrate density: gravel about 1.5 kg/L, sand about 1.6 kg/L, aquasoil about 0.8 kg/L.
• Substrate depth: 2.5–5 cm (1–2 in) for sand, about 5 cm (2 in) for gravel, 5–8 cm (2–3 in) for planted tanks.
• Water changes: 20–30% weekly for most community tanks; more for heavy stocking.
• Safe water quality: 0 ppm ammonia, 0 ppm nitrite, and nitrate ideally below 20–40 ppm.
• Schooling fish: groups of at least six, and more is better.