problem with black mollies

[Nimo]

I have a 29 gal fresh water tank that I am having some problems with. First of all I will tell you about the tank it has 3 black mollies 3 orange lear tail mollies, 1 regular mollie 1 balloon mollie, 3 miner tetras 3 male guppies 2 platties 1 peacock eel 1 clown pleco and a tiny bristle nose pleco.
Now I know I am well over stocked some of these fish will be getting moved to another tank soon. I am running a 30 gal tetra filter along with a 50 gal marieneland penguin biowheel, it has a heater and gets water changes 2 sometimes 3 times a week ammonia 0 nitrites 0 nitrates 5-10 ph 8.2. I have in the last week lost 3 guppies and one black mollie and there is another of my mollies not acting right, acts as if his back is broken, doesn't open his tail fin and is mostly using his front fins to get around. This is the same thing that the other mollie did before it died, what could be causing it?

[Nimo]

I wanted to add that I also have 4 other tanks of various sizes and none of them are having problems, 60 gal african cichlid tank, 2 ten gal and a 2.5 gal, none of which are showing any problems.