my fish are dying 1 by 1

[daydreams_uk]

My tank has been up and running for about 8 months now. The other day i noticed one of my mollies' fins were clamped tight to her body. Sadly she died. Since then i have had another4 fish die although they didn't show any symptoms whatsoever.
Could it be that i have an infection in my tank?

I have spent a fortune on my tank and fish and it's devastating me that the fish i have grown to love are dying one by one. Please could someone advise me on what i need to do to save the rest of them.
PLEASE!!!!!!

 

[dixaisy930]

How big is your tank, and what are your water perimeters?

 

[WILDER]

Clamped fins are a sign that they are stressed and that something is wrong, can you please post test results in ammonia,nitrite,nitrate,and ph.