How to mix reef saltwater in 7 steps
- Make your RODI water first. Use water that’s been through the DI stage, and note the TDS reading.
- Use a clean, dedicated container. Keep your salt and equipment away from detergents, metals and household residue.
- Get the temperature right. Bring the water close to the maker’s recommended temperature, usually around 24–25°C.
- Weigh the salt. Start with the label’s grams-per-litre figure for your target salinity. Don’t assume 35 g/L suits every brand.
- Add the salt to the water slowly. Run a small pump to keep the water moving, rather than tipping in a pile of salt in one go.
- Mix until clear, then measure. Use a calibrated seawater refractometer or another suitable salinity meter. Adjust in small steps with salt or RODI.
- Match it to your tank. Before a water change, the temperature and salinity should be close to your tank’s.
Worked example: 20 litres at 35 ppt
Red Sea publishes a dose of 38.2 g/L for Coral Pro at 35 ppt. For 20 litres, that’s 20 × 38.2 = 764 g to start with. Add it gradually, mix, measure and adjust. It’s a starting point, not a reason to skip the salinity check.
Three popular salts compared
| Salt | Pack | Checked price | Published yield | Good to know |
|---|---|---|---|---|
| D-D H2Ocean Classic Pro | 23 kg | £69.99 | 592–702 L | Suggested reef target of 35 ppt; designed for regular water changes |
| Red Sea Coral Pro | 22 kg | £74.99 | up to 660 L | 37.1 g/L at 34 ppt or 38.2 g/L at 35 ppt; higher alkalinity |
| Tropic Marin Pro Reef | 25 kg | £85.95 | Check the label on the bucket | Lower alkalinity; retailers list roughly 7–8 dKH |
How long to mix it, and how long it keeps
Every maker says something different. D-D says its H2Ocean salt is usually ready once clear, after about 30 minutes, and warns against mixing for longer than needed. Red Sea’s FAQ describes a controlled test mix and says not to aerate during the test. Aquaforest says its water can be used once clear, and that dissolved Reef Salt should be used within 3 days. Follow your own salt’s instructions rather than one rule for all.
Which salt should you choose?
Choose a salt to suit how you maintain your tank, not the adjectives on the bucket. If you rely on regular water changes, a balanced classic formula such as H2Ocean Classic Pro can suit you. A higher-alkalinity mix such as Coral Pro can suit a mixed reef or frag tank, as long as you use it consistently and keep testing. Tropic Marin Pro Reef is designed for lower, stable alkalinity. Switching salt can shift alkalinity, calcium and magnesium, so test after any change.
Sources and limits
We checked the mixing advice and prices against D-D H2Ocean information, Red Sea’s mixing FAQ, current Red Sea UK listings and a current Tropic Marin UK listing. We haven’t tested any salt batches ourselves.
Work out your salt cost per litre and yearly spend →
THE SHORT ANSWER
Start with tested RODI water, weigh the salt and check the salinity
You need clean RODI water, a dedicated container, a circulation pump and a calibrated salinity meter. Add the salt to the water, not water to a pile of salt, following the maker’s quantity, temperature and mixing time. The recommended dose is a starting point; always measure the salinity once the mix has settled.
Adjust slowly with a little more salt or RODI water, then match the temperature before you use it. Check alkalinity and other key parameters when you switch brands, open a new bucket or do a big water change. Not every salt can be heated or circulated overnight, so your salt’s own guidance comes first.
STEP BY STEP
How to do it
- 01
Measure the water
Fill the container with a known amount of tested RODI, leaving room for mixing and adjusting. Mark useful volume lines on the side.
- 02
Calibrate your salinity meter
Use a suitable reference solution and clean the prism or probe. How you rinse it and handle temperature affects how consistent your readings are.
- 03
Add the salt and mix
Weigh the starting amount, add it gradually to moving water, and follow the salt’s recommended temperature and mixing time.
- 04
Check and adjust
Measure the settled salinity, correct it in small steps, and note how many grams you used for that batch and target.
YOUR SITUATION
What to do in your situation
Same salt and the same container volume as usual
Use your recorded recipe, but still check the salinity.
The chemistry and yield may be different
Mix a test batch and measure salinity plus the key parameters.
Speed matters, but a chemistry mismatch is risky
Use water you prepared earlier, check the essentials and avoid untested shortcuts.
COMMON MISTAKES
Mistakes to avoid
Adding water to dry salt
A very concentrated patch can cause precipitation and uneven mixing.
Calibrating with pure water
Many refractometers should be calibrated close to the range you’re measuring, using a proper reference fluid.
Using new water to correct the tank
Deliberately mixing extreme salinity or alkalinity can cause sudden swings. Handle corrections separately, and calculate them.
WORKED EXAMPLE
Building a repeatable 25-litre recipe
The container is marked at 25 litres and filled with tested RODI. The refractometer is calibrated with the right reference fluid, and the reef keeper weighs out slightly less salt than their last complete batch needed. The salt goes in gradually while the pump runs, following the salt’s instructions.
After the recommended mixing time, the salinity is a little low, so they weigh a small extra amount, let it dissolve and test again. They note the final total. That recipe speeds up the next batch, though measuring still has the final say.
BEFORE YOU CHANGE ANYTHING
Write down where you’re starting from
Note the date, the equipment or product you’re using, how you measured and the conditions behind every important number. Say whether a volume is the tank’s label figure or your real net water; whether a flow is the box rating or what actually comes out; and, for test results, the salinity, test kit and time of day. A precise-looking number is useless once you’ve forgotten where it came from.
Change one thing at a time, and decide in advance what improvement will look like: a steady test trend, less detritus, a more stable temperature, a quieter tank or a lower running cost. One reading, or one good day, isn’t a trend, and a setting copied from someone else’s tank isn’t proof it suits yours. The free Reef Companion keeps your tests, maintenance and changes in one dated log, so you can see cause and effect.
MAKING CHANGES SAFELY
Protect your tank while you make the change
Take a baseline
Measure before you install equipment, change your chemistry or alter your routine. Take photos if placement matters, and note how your livestock behaves. Without a starting point, you can’t be sure what caused any difference later.
Start small
Begin with a cautious setting or dose, and keep a way back. Hang on to the old equipment until the new one has proved itself, keep your previous schedule, and don’t add livestock at the same time as a big lighting, flow or chemistry change.
Give it the right amount of time
Mechanical problems show up straight away; nutrient and coral responses can take days or weeks. Judge the change over a sensible period, but stop immediately if you see animals in distress, a leak, overheating or an electrical risk.
Afterwards, note the result and decide whether to keep, adjust or undo the change. If the result is unclear, go back to your last stable setup and measure better, rather than piling on another change.
SIX MONTHS ON
Check it’s still the right decision
What made sense on day one can stop working as livestock grows, the room warms up, equipment gets dirty or a routine turns out to be unrealistic. Set a reminder to review it in six months, comparing what you assumed with the reality: water volume, electricity, consumables, noise, cleaning time and test trends.
Test trends, real flow, temperature, light coverage or whatever the change was for
Is it still doing its job, with room to spare?
How often you clean it, how hard it is to reach, calibration drift and missed tasks
Can you keep this up without cutting corners?
Electricity, reagents, filters, spare parts, warranty and downtime
Would you make the same choice knowing the real cost?
If it’s working, you don’t need to replace it; the review simply gives you a reason to keep it, tune it or plan an upgrade before something fails.
KNOW THE LIMITS
When to get expert help
Get product-specific or professional help for anything involving floor loading, household electrics, unsafe plumbing, damaged glass, unexplained livestock deaths, or chemistry you can’t measure reliably. Use a qualified electrician for fixed wiring, and always follow the current safety instructions for your tank, equipment and additives. Never work around damaged cables, swollen batteries, leaking seals or equipment that keeps tripping the electrics.
Your livestock’s needs can also outweigh general equipment advice. Check adult size, behaviour, disease risk and each species’ light, flow and feeding needs with reputable care guides before you buy. This guide explains the equipment decision; it can’t guarantee the result for every animal or tank.
COMMON QUESTIONS
Frequently asked questions
How much salt should I add per litre?
Use your salt’s guidance as a starting point and check the final salinity; formulas and target salinities differ.
How long should reef salt mix for?
Follow the maker’s instructions. Some salts are ready quickly; others need a different time.
Can I mix reef salt with tap water?
RODI gives you a far more controlled starting point. Tap water varies and can bring in unwanted substances.