With bulging eyes, there is one key question to ask: is it one eye swollen, or both?
If just one eye, the problem is likely physical damage. Swelling behind the eye after an injury causes the eyeball to protrude. Given good water conditions, the fish should recover under its own steam. The use of a little Epsom salt (magnesium sulphate) at 1 teaspoon per 5 to 10 gallons can help reduce the swelling.
If both eyes are swollen (even if by different amounts) the problem is almost certainly water quality. Mollies are extremely sensitive to poor water quality, and one of several reasons why they are best kept in slightly brackish water is that salt detoxifies nitrite and nitrate to a degree. Mollies also need much warmer water than most tropical fish. Neons for example should be kept between 22-25 degrees C, but mollies need around 28-30 degrees C, so you can't keep these two species together. When they're too cold, or exposed to poor water quality, mollies suffer from a variety of ailments, bulging eyes (exophthalmia) being one of them.
So, to recap, mollies need hard water (15+ degrees dH) with a basic pH (7.5-8.2). The temperature should be fairly water (28-30 C). Water quality should be excellent; 0 ammonia, 0 nitrite, and as close to 0 nitrate as possible. Ideally, you'd also add about 5-9 grammes of marine salt mix per litre of water. Note that "teaspoon per gallon" amounts of salt are useless, and plain vanilla tonic salt far inferior to marine salt mix.
Cheers, Neale
my molly has got swollen eyes and stays at the bottem of the fish tank, it's still alive but i don't know what to do, can anyone give me any suggestions on what to do? thanks