Salt is commonly added to livebearer tanks because many of the livebearer species were originally collected/discovered in slightly salty water.
However, platys are freshwater and don't like salt in the water. Mollies like salt in the water. That's one of the reasons they shouldn't be kept together. (The salt softens the water, to a degree.)
to give the brain fresh air!
If I had fish (platys) lying on the bottom of my aquarium I would be checking the water temp and quality. There must be something wrong. A water change will probably help but don't forget to equalize the temps and other parameters otherwise you may shock and kill your fish. I agree with the above. They need warm water (not room temp), about 72-82 degrees.
It makes your breath smells FREsH. For other people it just a habit.
Moldy cheese or an eyeball
yes. please provide tips on fresh water aquarium fish too.
As a decoration, maybe. As a living entity--NOPE.
You can raise a sea urchin in a salt-water aquarium. There are no fresh water sea urchins so you would need a salt-water aquarium to properly care for the urchin.
Guppies, Platties, Swordtails and Mollies. there are a few others, but you likely won't see them in the aquarium trade.
Yes but use the proper dosage of aquarium salt for the size of your tank. You might look into a brackish aquarium guide.
Done frequently, and easily with many species.
100 degrees or maybe in the 70 ranges
Nothing...they die....