not being nasty, but i would put the molly in a seperate/quarintine tank, and then totally sort that 20G tank out, redo it all.
take the water out, plants, substrate, rocks/ornaments.
Idea:
Sand with gravel markings... i.e sand with gravel paths
some nice tall plants, not sure on the plant side of things, ask someone for suggestions
1 big homemade/store rock cave
1 small homemade/store rock cave
air pump?
*whats that white stuff near the filter?* get a new filter.
The Fish:
Obviously put back your original Molly, and by the looks of things you have a male, correct me if im wrong.
3 Female mollys
6 guppies? 2male 4female
10 neon or cardinal tetras
3-6 corys of any kind, i personally like the Three-Line corys.
a few shrimp...
and that should do you...
you could get a couple of betta's 1 male 1 female of course...
and be prepared for eggs and fry
make sure you have 1 or 2 5 gal tanks for quaritine and breeding/fry