tropical fish dying

Question
i have had my 60 liter tank set up since Christmas, i haven't added any fish in about a month. i use a gravel vacuum and do 40% water changes weekly, i have had no problems so far until a few days ago when i noticed my tank was a tiny bit cloudy. i have 2 gouramis, a siamese fighter, 6 neon tetras, 7 mollies and 2 rosy barbs. my siamese fighter and all the mollies have died but all the others act and look healthy. the pH has always been fine but i tested it after 1 died and it had suddenly gone up for no reason, so i have done another water change and it is back to normal. the mollies kept duing and my local shop doesnt know why either. can you help?

Answer
Dear Daniel,

  I'm not real sure about your problem, but its a possibility that there is to much of a biological load.  Your tank is not overstocked, but it seems that there just isn't enough bacteria to support it yet.  
  I would keep doing water changes but try doing 20% - 25% a week and then when you see some algae growth try adding some Mollies.  
  I hope this helps you some.