Matt Clarke answers some of the most common questions on how and why salt is used when fish are being transported, and explains what should be done when they arrive.
Why is salt added when fish are being shipped?
Freshwater fish are saltier than the water they live in and their skin is semi-permeable.
The concentration gradient between the freshwater and the fishes' saline innards mean that they leak salts into the water and need to pump them back in to their bodies (via special cells in the gills) to keep their bodies salty. This is called osmoregulation.
When freshwater fish get stressed, as they can do when they're being flown across the world in a bag, they leak bodily minerals into their water.
Research has shown that adding salt to their transport water can minimise the amount of salts they lose, which in turn reduces stress.
This can dramatically reduce the number of fish that die on the journey and should mean that the fish are less likely to develop diseases when they're being quarantined.
Does salt reduce stress?
Yes. Several studies have shown that many fish, even those that never live in salty water, have lower stress levels and higher survival rates during and after transport when salt is present in their water. For these reasons, salt is widely used during transit for fish, both in aquatics and in aquaculture.
In 2001, two Brazilian scientists, Caneiro and Urbinati, studied the stress levels of a big, predatory characin called Brycon cephalus during transport.
The fish were shipped in water containing different concentrations of salt and their blood was sampled during the journey to see how much stress they were under.
By analysing the blood of the fish and monitoring the levels of chemicals and hormones which are produced under stress, the scientists were able to determine what effect the salt was having on their health.
When the fish were packed in unsalted water, or in water containing just 0.1g per litre of salt, analysis showed that the stress hormone plasma cortisol had built up, indicating that the fish had been stressed by the journey.
However, when the fish were packed in water containing 3ppt salt, this hormone wasn't produced.
At a dose of 6ppt, the blood glucose levels of the fish stayed the same, so Caneiro and Urbinati reckon that 6ppt is the best dose for shipping this species.
Brycon cephalus comes from the very soft, acidic waters of the Amazon basin. Most people wouldn't normally consider fish from this sort of environment as being particular tolerant of salt!
Another scientist suggested 2ppt as a guideline, but most Singapore suppliers are said to dose their fish with salt in transport at a rate of 0.5-3ppt.
Some studies have found that very high levels can have remarkable effects, with some species reportedly shipping well in water containing doses as high as 9ppt. This is close to isotonic.
What does 'isotonic' mean?
Older PFK readers may remember an early 1990s' TV advert for Lucozade Sport drink featuring Liverpool footballer John Barnes. His end quote was: "It's in balance with your body fluids." And that's what isotonic means, in a nutshell.
Freshwater fish usually have an internal salt level of about 10ppt, so if the water in your tank is also 10ppt, it's said to be isotonic. At isotonic levels, the water is about as salty as the fishes' innards, so it won't lose much salt at all.
However, isotonic levels in transport may well be a bit over the top for many fish, and they're not really designed to live in water of this salinity for long, so such levels aren't advisable.
Surely Discus, Cardinals and other wild-caught fish aren't salted?
Actually, many are. In 2001, Waichman, Pinheiro and Marcon studied the techniques used by fish suppliers operating on Brazil's Rio Negro, where many Discus, Cardinals and Hatchets and other species are imported from.
Their study showed that salt was used when the fish came from the first holding station upriver and at the exporter's facility in Manaus.
However, here the authors guessed that it may, in fact, cause more harm than good as these fish come from such incredibly mineral deficient water.
Should newly imported fish be kept in salted water?
In some cases, yes, and this is more important than many people realise. If the fish have been imported in salted water, research has shown that losses are lower if they are added to salted tanks and then gradually acclimatised back to freshwater over a period of days.
For example, Lim et al (2002) shipped guppies from Singapore in water with 1, 3 and 9ppt of salt, and then placed the fish in either freshwater or water containing the same level of salt to their transport water.
When the fish were added to freshwater, the level of losses in all batches were about the same.
However, when they were placed in tanks containing the same salt level to their transport water, and then acclimatised back to freshwater via a 30% daily water change, losses were greatly reduced!
Lim reckons that the addition of salt, even only 1ppt, is critical to the recovery of guppies after transport.
Therefore it's important that your dealer, or his wholesaler, puts newly imported fish into salted tanks (if they arrived in salted shipping water) to start off the quarantine process.
Lim calls this 'recovery water'. The fish then need to be gradually acclimatised to freshwater through daily 30% water changes to dilute the salt.
There is no need for the shop, or you, to keep guppies or other non-brackish species in salted water permanently. Indeed, none of the experts we spoke to advocated long term salt use.
Besides, guppies are freshwater fish, anyway.
My dealer says he tested the water in the fish bags and has matched his chemistry to that. Good idea?
Possibly not, as he may be measuring the salt in the transport water. Similarly, it's not wise to match the pH.
Carbon dioxide builds up in the bag on the journey which causes the pH to plummet.
If the pH shows a very low level, this might not, in fact, be the pH of the water prior to the journey. Speaking to the supplier is best, but that's rarely possible.
How should I acclimatise the fish when I get them home?
Since the fish will be physiologically adapted to life in water that may be different to yours, you'll need to acclimatise them slowly and carefully to minimise any shock.
It's often recommended that new fishkeepers simply float bags of new fish on the surface of the tank for half an hour and then gently release them.
In my opinion, this isn't the best way to do things. If you float a bag of water that's 23C in an aquarium of 25C, it will take less than a couple of minutes for the bag water to equilibrate. Try it if you don't believe me.
Since fish can't get used to any changes in chemistry through a plastic bag, the other 28 minutes of floating serve no purpose whatsoever. Tip the fish straight into the tank and the rapid change in chemical concentrations will shock them.
Instead, I believe it's important to slowly add aquarium water to the water in the bag from the shop over a fairly long period of time.
If you put a small cupful of water into the bag every five minutes for 45 minutes, the fish will gradually be introduced to the type of water in your tank.
It is true that fish do not become fully adjusted to changes in the water for some time, but in practice, this definitely works. No dealer would dream of introducing new fish without getting them used to the water beforehand.
If your dealer sells his fish from salted tanks, take care when acclimatising them. We wouldn't advise you to try and maintain a low level of salt in your tank, unless the fish are brackish. Slowly get them into freshwater instead.
This is an item from the Practical Fishkeeping archives.