Aquarium Volume Guide: Litres, Gallons, Filter Size and Heater Watts

Tank volume in litres is length × width × height in centimetres, divided by 1,000. A 90 x 45 x 45 cm tank works out to 182 litres of glass. The water in it will be closer to 155 to 165 litres once you allow for substrate, rock, decor and the gap below the rim.

That second number is the one you dose medication with, and getting it wrong is a common way to kill a tank.

Common Australian tank sizes

Realistic volume assumes a 15% reduction for substrate, hardscape and fill level. A bare-bottom tank loses less.

Gross volume versus real water volume

The litres printed on the box is the volume of the box. Your actual water volume is lower for three reasons:

• Substrate. A 5cm sand or gravel bed in a 90 x 45 tank displaces roughly 20 litres.
• Hardscape and decor. Rock, wood and ornaments displace their own volume. A decent rock scape easily takes 10 to 15 litres out of a 180L tank.
• Fill level. Nobody fills to the top rim. There's usually 3cm to 5cm of air, which in a 90 x 45 tank is another 12 to 20 litres.

A reasonable rule is subtract 10% to 15% from the gross volume for a planted or aquascaped tank. If you want to be exact, fill it with a bucket of known volume and count.

Why the number decides everything else

Filter turnover

Filters are rated in litres per hour. The standard target is 4 to 6 times the tank volume per hour for a normal community tank, and 8 to 10 times for goldfish, cichlids or anything heavily stocked.

A 180L community tank wants roughly 720 to 1,080 L/h. Two things to know: manufacturers measure flow with an empty canister and no head height, so real flow with media and a metre of hose is often 30% lower, and a filter that's slightly oversized is never a problem while an undersized one always is. Buy above the number.

Heater wattage

Rough guide is 1 watt per litre if the room sits within about 5°C of your target temperature, and 1.5 watts per litre for a cold room or a Melbourne or Hobart winter with no house heating.

A 180L tank in a normal Sydney lounge room wants around 200W. In an unheated Tasmanian sunroom, closer to 250W to 300W. Two 150W heaters at opposite ends beat one 300W heater, because if one sticks on it can't cook the whole tank, and if one fails you're not at room temperature by morning.

Dosing

Dechlorinator is dosed on the volume of new water going in, not the whole tank. A 25% change on a 160L tank is 40L of new water, so you dose for 40L.

Medications and fertilisers are dosed on actual tank water volume. Dosing a 160L tank as though it holds 182L is a 14% overdose. For most fertilisers that's survivable. For copper-based treatments and some antiparasitics it isn't.

Water changes

25% weekly is the standard starting point. On a 160L tank that's a 40L change, which is four 10L buckets, which is why people buy a python-style hose after about three weeks of tank ownership.

Litres to gallons, and the trap in American videos

Australia runs in litres, but most aquarium content on YouTube and Reddit is American and uses US gallons. There are two different gallons:

• 1 US gallon = 3.785 litres
• 1 imperial gallon = 4.546 litres

So a "55 gallon" tank in an American video is 208 litres, not 250. A "10 gallon" quarantine tank is 38 litres. Getting this wrong by using imperial gallons inflates every dosing and stocking figure by 20%.

Weight: check the floor before you fill

Water is 1kg per litre. Add the glass, the stand, the substrate and the rock.

• A filled 60 x 30 x 30 (54L) tank: roughly 80kg
• A filled 90 x 45 x 45 (182L) tank: roughly 250kg
• A filled 120 x 50 x 50 (300L) tank: roughly 400kg

A 400kg load sitting on a stand footprint of about half a square metre is a serious point load on a timber floor in an older Queenslander or a first-floor apartment. Run big tanks across joists rather than along a single one, and near a load-bearing wall if you can.

Stocking: ignore the inch-per-gallon rule

"One inch of fish per gallon" is one of the worst rules of thumb in the hobby. It treats a 10cm goldfish and ten 1cm neons as the same load, which they aren't. A goldfish produces vastly more waste and needs vastly more swimming space.

What actually matters:

• Adult size, not shop size. The 4cm fish in the tank at the shop might be a 30cm fish in two years.
• Bioload. Messy eaters like goldfish, oscars and plecos need far more filtration per centimetre than tetras.
• Footprint over volume. Active swimmers need length. A tall 100L tank is worse for rainbowfish than a long 80L one.
• Schooling minimums. Most schooling species need six or more, which the volume has to allow for.

Before you add fish

Cycle the tank first. Ammonia converts to nitrite, nitrite converts to nitrate, and that bacterial colony takes roughly 4 to 6 weeks to establish without fish in the tank. Adding fish to an uncycled tank is the single most common reason a new setup fails.

Australian tap water is chlorinated or chloraminated depending on where you live, so dechlorinator isn't optional. Check with your local water utility if you want to know which, because some conditioners handle chloramine better than others.

Related calculators

• Aquarium Volume Calculator for litres and gallons from your dimensions
• Unit Converter for anything else in centimetres, litres or imperial
• Pet Cost Calculator if you're budgeting for the fish as well as the tank

Frequently asked questions

How do I calculate aquarium volume in litres?

Multiply length by width by height in centimetres and divide by 1,000. A 90 x 45 x 45 cm tank is 182 litres gross. Subtract 10% to 15% for substrate, decor and the fill line to get your real water volume, which is about 155 litres.

How many litres per hour should my aquarium filter be?

Four to six times the tank volume per hour for a normal community tank, and eight to ten times for goldfish, cichlids or heavily stocked tanks. Rated flow is measured without media or head height, so real flow is often 30% lower. Buy above the number.

What size heater do I need for my tank?

About 1 watt per litre if the room stays within 5 degrees of your target temperature, and about 1.5 watts per litre for a cold room. A 180 litre tank usually wants 200W to 300W. Two smaller heaters are safer than one large one.

How many litres is a 55 gallon tank?

208 litres, using US gallons at 3.785 litres each. Almost all American aquarium content uses US gallons. Imperial gallons are 4.546 litres, and mixing the two inflates every figure by about 20%.

How much does a full aquarium weigh?

Roughly 1kg per litre of water, plus glass, stand, substrate and rock. A filled 182 litre tank is about 250kg and a 300 litre tank is about 400kg. Check the floor structure before setting up anything large upstairs or in an older timber-floored house.