Poorly Fish Needs Help!

[stotty1001]

I have a tropical fish tank that has been set up for aprox 6 weeks now. I had 2 white Mollies. One died a few days ago. She started to dart about the tank and go into a spasm. She then started having trouble keeping herself up near the surface and started sinking to the bottom of the tank. She then turned upside down and was breathing very slowly. Not long after she passed away.
The Mollie that I have left has just had a spasm and is acting very strange. Im very concerned that it will have the same fate as my other fish. I have checked nitrate levels and made sure the water is acceptable. I need some advice as to what may be the problem.

I hope someone can help before its too late.
Thanks

 

[samiam9818]

what are your tank stats? (ammonia, nitrIte, nitrAte, Ph)

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

How old were the fish.
Fish can dart, heavy breath, and turn upside down when there dying.