I have noticed that Neale promotes the use of salt as a tonic for common livebearers and must say that I find the concept quite foreign.
Neale absolutely does NOT do this! I'm quite against the idea of "tonics" in fishkeeping!
My argument about mollies is this: I agree they don't come from brackish water for the most part, and wild populations seem just fine in freshwater. But my experience at Wet Web Media is that a LOT of people have problems keeping mollies alive for any length of time. For casual aquarists, the fact marine salt mix raises carbonate hardness and steadies a basic pH around 7.5 is useful. By telling them to add a little marine salt mix they can create conditions that mollies thrive in. That there's also salt in the marine mix is somewhat secondary, though sodium chloride does reduces nitrate and nitrite toxicity, and that in itself may be very helpful where inexperienced aquarists are concerned. So purely as a practical, humane approach, the use of marine salt mix -- and note I stress the use of marine salt mix, not plain aquarium salt -- is I believe the quickest and easiest way to help beginners keep their mollies happy.
As for guppies, I can see both sides of the argument. But again, more often than not, where wimpy fancy guppies are concerned, raising the temperature and adding a little marine salt mix can nudge conditions towards those that ensure better health. Is it essential? Absolutely not. Do guppies need to be kept in brackish water? No. Am I arguing simply for the use of salt as a tonic? No. It's the fact marine salt mix raises hardness and pH that I believe is of prime value, and the detoxification of nitrite and nitrate a secondary one. But this is all based on science, and not some hand-waving idea about slime coats or osmotic balance or any such thing!
The fact your mollies and guppies are healthy in cooler, freshwater conditions means -- in my opinion -- you're a good fishkeeper. You know what you're doing and you're ensuring good conditions via water changes, proper filtration, a healthy diet, and appropriate sized aquaria. Like you, I keep my livebearers in non-brackish conditions. I have both
Ameca splendens and
Limia nigrofasciata happily breeding away in moderately hard, slightly basic water -- a 50/50 mix of tap water and rainwater. Indeed, the latter species are in the same tank as my
Nomorhamphus halfbeaks, and both are breeding happily.
But my advice here and in magazine articles is, when it comes to mollies, that less experienced aquarists can "play it safe" by using marine salt mix and slightly warm water conditions, certainly warmer than platies and swordtails enjoy. Put another way, creating slightly brackish conditions doesn't harm guppies or mollies, so there's nothing to lose. Keep those guppies or mollies in plain freshwater, and many aquarists find they get sick. It's a no-brainer for me: add a little marine salt mix, and see if that helps!
Cheers, Neale