OK, lets clear up the Mollie thing.
Mollies do not prefer salt in itself, rather they prefer the water conditions that come along with it.
Mollies prefer very clean water, no nitrite, very little nitrate (levels as much as 5ppm may be too much for them). The prefer warm, hard and alkaline water.
Marine Salt mix can provide ideal conditions for Mollie, hence they're 'tolerance' of salty water. Because some are found wild in brackish they are still happy in perhaps full seawater.
Tonic salt is useless for Mollies, other than it can help protect them from Nitrites.
So, in summary, Mollies are harder to keep than people think, but marine salt can help.