Help Fish Dieing

[bosinc1]

I need help. I have a 55 gallon tank and I 12 fish in tank. I had did water change 2weeks ago and in the last 4 days I have lost 4 and water test have been perfect. No change in water temperatures.
I have clownloach, 2 fancy mollies, 3 beta's, tetra, 2 algae eaters, 3 dwarf grummies, I have lost 1 grummy, 3 bets' s, 1 Molly with in the last week. Went to pet store and they don't understand why I am loosing fish with everything test perfect.
Note I also had a white film on top of water that no one could figure out where it came from. It appeard about 2 months ago even after water changes but after the last two fish died the film has gone away. I don't understand what is going on. Tank has been up for about 8 months now

 

[AshleyNZ]

Um, im guessing by "grummy" you mean Gourami? 
First off, let me tell you that your tank is too small for the clown loaches.
Bettas should not be housed together, unless it is an over stocked amount of females to reduce aggression. Bettas of either sex should not be housed with gourami. 
Please tell us the specific numbers of ammonia, nitrates and nitrites to help us understand the situation a bit better. Sounds like you don't do waterchanges very often either, they should be completed at least weekly. 

 

[Byron]

I agree with Ashley on all points.  It would also help if you can give us the water parameters, namely GH (general hardness) and pH and temperature.
 
The whitish film might be a protein scum; this is often seen on the surface and generally it is harmless, but it can sometimes thicken and this can reduce the exchange of oxygen/CO2 at the surface.  Increasing slightly the surface movement (by adjusting the filter return) can sometimes prevent this.  Of course, it may be something completely different.  But a weekly partial water change will also help.
 
Byron.