How Much Saltwater Do I Need? Aquarium Fill Calculator (UK)

How much saltwater do I need? Multiply your tank’s internal length × width × water depth in centimetres and divide by 1,000 to get litres, add the sump at its running level, then subtract 10–20% for rock and sand. A typical 245-litre all-in-one reef tank needs about 200–220 litres, or eight to nine 25-litre containers.

This guide is for anyone filling a new marine tank or planning a re-fill. It gives you the formula, a ready-made table for popular all-in-one reef tanks sold in the UK, and practical first-fill tips so you order the right amount the first time.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• Litres = length × width × water depth (cm) ÷ 1,000; for gallons, divide litres by 4.546 (UK) or 3.785 (US).
• Live rock, dry rock and sand typically displace 10–20% of the water, so a first fill needs about 80–90% of the rated system volume.
• Use the total system volume (display plus sump) for all-in-one tanks, not just the display figure.
• Order one extra 25-litre container beyond your calculation for topping up the sump and your first water change.

How do you calculate how many litres your tank holds?

For a rectangular tank, measure the inside of the glass, not the outside, and use the height of the water rather than the height of the glass.

1. Measure in centimetres: internal length, internal width and water depth (usually 2–3 cm below the top rim).
2. Multiply and divide by 1,000: L × W × H ÷ 1,000 = litres. Example: 90 × 50 × 47 ÷ 1,000 = 211.5 litres.
3. Add the sump at its normal running level, not when brim full. Leave room for water that drains back from the display when the return pump stops.
4. Subtract rock and sand: take off about 10% for a light aquascape and thin sand bed, up to 20% for lots of rock or a deep sand bed.
5. Round up to whole containers: divide by 25 for the number of 25-litre containers, then add one spare.

Bow-front, corner and cylinder tanks are harder to measure. The manufacturer’s figure is the easiest starting point, or count the containers on the first fill and write the number down for next time.

How much saltwater do popular all-in-one reef tanks need?

Rated volumes below are manufacturer or UK retailer figures (Red Sea’s figures are from its Reefer G2+ specifications). The fill column allows 10–20% for rock and sand. Newer generations of the same models may differ slightly, so check your manual.

Tank	Rated system volume	Likely first fill	25 L containers
Red Sea Max Nano Cube G2	75 L	60–68 L	3
Waterbox AIO 20 Cube	76 L (display)	61–68 L	3
Red Sea Max Nano Peninsula G2	100 L	80–90 L	4
Red Sea Max Nano XL G2	125 L	100–112 L	4–5
Red Sea Reefer 170 G2+	165 L	132–148 L	6
Red Sea Reefer 200 G2+	200 L	160–180 L	7–8
Red Sea Reefer 250 G2+	245 L	196–220 L	8–9
D-D Reef-Pro 900	290 L	232–261 L	10–11
Red Sea Reefer 300 G2+	300 L	240–270 L	10–11
Red Sea Reefer 350 G2+	342 L	274–308 L	11–13
Red Sea Reefer 425 G2+	433 L	346–390 L	14–16
Red Sea Reefer 525 G2+	540 L	432–486 L	18–20

If your tank is not listed, use the formula above. And remember the numbers in this table are for the first fill only. Ongoing needs depend on your water-change routine, covered in how much and how often to change water in a saltwater aquarium.

Why does rock and sand change how much water you need?

Rock and sand take up space that would otherwise be water. Older hobby rules of thumb suggest about half a US gallon (roughly 2 litres) displaced per 10 lb (4.5 kg) of rock, and about three-quarters of a US gallon (roughly 3 litres) per 10 lb of sand, but porous rock varies widely. That is why the 10–20% range is a sensible planning figure: it stops you over-ordering heavily while leaving enough spare to finish the fill.

Equipment matters too. A skimmer, reactors and return pump in the sump all displace water, and a chiller or UV adds pipework volume. If in doubt, round up: spare saltwater is never wasted, because you will use it for your first water change.

Should you fill a new tank with natural seawater?

You can fill a new marine tank with synthetic salt mixed in RO/DI water or with natural seawater. Natural seawater comes ready to use, so there is no week of producing RO water and mixing batch after batch, and it carries natural bacteria, phytoplankton and trace elements. Typical parameters of ours are salinity 34–36 ppt, calcium 400–450 ppm, alkalinity 7.5–8.6 dKH and magnesium 1250–1350 ppm, which sit right in the ideal range for marine tank water.

being NSW ive not had to wait for the tank to cycle either

Gary, from our customer reviews

That is one customer’s experience. We still recommend testing ammonia and nitrite and adding livestock slowly, as set out in our step-by-step reef tank setup and cycling guide. For the wider case, see why reefkeepers choose natural seawater.

First-fill tips: how to fill a saltwater tank

• Check the stand and tank are level and the room can take the weight: 25 litres of seawater weighs about 25.6 kg, so a 245-litre system holds roughly a quarter of a tonne of water.
• Fill slowly onto a plate or bag placed on the sand to avoid a sandstorm, or fill before adding sand if you are using dry sand.
• Fill the display first, then the sump to its running level with the return pump off. Switch on, then top up the sump to the marked line.
• Match temperature to 25–26 °C with heaters running before adding anything alive.
• Check salinity with a calibrated refractometer once the water is circulating: 35 ppt (SG 1.026) is the reef standard.
• Keep RO/DI water for top-off. Evaporation removes fresh water only, so top up with RO/DI, not saltwater.

Carrying 25 kg containers up stairs is the hardest part of a first fill. Our aquarium saltwater delivery service can pump water straight into the tank. For every buying option compared, read where to buy sea water for an aquarium.

Frequently asked questions

How much saltwater do I need for a 200 litre tank?

Allowing 10–20% for rock and sand, a 200-litre system needs about 160–180 litres of saltwater for the first fill. That is seven or eight 25-litre containers, and ordering one extra container gives you water for topping up the sump and your first water change.

How many 25 litre containers do I need to fill my tank?

Divide the litres you need by 25 and round up. For example, a 245-litre all-in-one needs about 200–220 litres after rock and sand, which is eight or nine 25-litre containers. Add one spare for the sump and your first water change.

Should I include the sump when working out how much saltwater I need?

Yes. Use the total system volume, which is the display plus the sump at its running level. Leave space in the sump for water that drains back from the display when the return pump switches off, or it may overflow during a power cut.

How much does live rock displace in a saltwater tank?

As a rough guide, rock and sand together displace about 10–20% of a tank’s volume. A lightly aquascaped tank with a thin sand bed sits near 10%, while heavy rockwork or a deep sand bed can reach 20% or more.

Can I fill a new aquarium with natural seawater?

Yes. Natural seawater can be used for the first fill and arrives ready to use with typical salinity of 34–36 ppt. You should still test ammonia and nitrite and add livestock gradually, and our delivery service can pump the water straight into your tank.

Sources

External sources cited in this article (checked October 2026):

• Reefer G2+ specifications (Red Sea)

Bottom line

To work out how much saltwater you need, calculate the litres of your display and sump, take off 10–20% for rock and sand, and round up to whole 25-litre containers plus one spare. For a fast, low-effort start, natural seawater can be delivered and pumped straight into your new tank, and new customers can try 25 litres free.