Checking salinity is a quick but important part of managing your lobster tank

Salinity is one of the most important water parameters to monitor when holding live lobster, crab or langoustine. Around the UK coast, natural seawater is typically between 32 and 35 parts per thousand (ppt) and this is the range we work with in our live shellfish holding systems. Checking the salinity regularly helps you make sure the water remains where you expect it to be.

In the short video above Todd Fish Tech’s marine biologist Dr Keith Todd shows you how to check salinity using a refractometer.

What salinity should a lobster tank be?

For our live shellfish holding system seawater salinity should be 32–35ppt similar to the natural seawater found around the UK coast.

If you’re collecting natural seawater it’s still worth checking it before use rather than assuming the salinity is correct. If you’re making artificial seawater, checking the finished mix lets you make any adjustments before adding it to your system.

What do you use to measure salinity in a lobster tank?

At Todd Fish Tech we use a salinity refractometer.

A hydrometer can also be used but we prefer a refractometer because hydrometers tend to be more fragile and, in our experience, less reliable for routine testing.

A refractometer is small, simple to use and gives you a quick reading in parts per thousand.

How do you use a salinity refractometer?

Take a small sample of the seawater and place it onto the slide of the refractometer. Hold the refractometer towards the light and look through the viewing port.

You’ll see a scale inside which allows you to read the salinity of the water in parts per thousand.

In our video Keith tests a sample of natural seawater and gets a reading of 32ppt,which is within the range we’re looking for.

It takes less than a minute to do as Keith demonstrates.

What if the salinity is too high or too low?

If you’re making artificial seawater and your salinity is too low more marine salt can be added to your mix. If the salinity is too high it can be diluted using suitable fresh water.

If you’re using tap water remember that it may contain chlorine or chloramine and needs to be appropriately treated before being used in a live shellfish holding system.

We’ll cover making artificial seawater and managing high salinity in more detail in separate Todd’s Top Tips.

How often should you check the salinity in a lobster tank?

Salinity can change over time particularly as water evaporates. The water disappears but the salt remains gradually increasing the concentration of salt in the system.

Keith recommends checking the salinity one a week so you can spot if it has started to creep upwards.

It’s a very quick test and one worth making part of your normal water-quality checks.

Need help with your lobster holding system?

Todd Fish Tech has been designing and manufacturing live shellfish holding systems in the UK since 1998.

If you’re an existing Todd Fish Tech customer and need some help understanding your water-quality readings please get in touch. If you’re considering lobster tanks for your business we’d be happy to help you work out the right setup.

Last updated September 2026