Hello again sev...
This is a good conversation. We may be "splitting hairs", but your reference to undergravel filters versus the filters we have now is exactly what you've described, improvements in technology not in chemistry. Salt works the same way it did then as it does now and will in the future. It can't be improved upon, where technology is always getting better.
I have no doubt you have considerable experience with fish keeping and so do other establishments that use salt in their freshwater tanks and have for decades. An example of what I'm talking about is Aquatics Unlimited in Milwaukee, Wisconsin, a large breeder of freshwater fish. They've used salt in their freshwater tanks, 12,000 gallons of freshwater tanks, since the 1960s. They use about three times more than I'd use, but how do you argue with success. They obviously know what they're doing.
Thanks for you post.
B
They clearly do know what they're doing. Salt as a prophylactic works wonderfully for a few days on fish for quick turnaround since it subdues (not cures) many common ailments that come in with new fish from even the best distributors and breeders. However, as a long term (in the perspective of the fish, not the tank or store) it generally serves no purpose and can pose problems for quite an array of fish if long term goals, breeding, etc are the pursuit. Salt as a tonic does more damage because people think it is a "medicine". Very few fish truly do their best in slightly brackish water. Even less if they use aquarium salt. If a fish comes in with a disease, the most ethical practice is to attempt to diagnose and cure the cause. Accurately and without tonics. JMO