http://www.practicalfishkeeping.co.u...article_id=392
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.
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.
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.