Please help ... I am very concerned about a poor molly in my tank

[kbdionis]

Hello, everyone. I am hoping someone can help me. I am fairly new to this site and fish keeping in general.
I have been very lucky and have encountered no problems with my 30 gallon brackish tank until now.
I have five mollies and two dwarf puffers in the tank. I am hoping to maybe get a scat and a mono.
I have one molly who has not thrived at all. The other mollies are at least twice their original size. However, they do not seem to bother the small molly I am referring to.
This molly appears to be shimmying and has a probem staying horizontal. The molly holds her fins very close to her body. I feel absolutely terrible for the poor little girl.
The fish swims desperately but barely moves forward. She cannot stay horizontal and often rests on a rock.
I have tried to figure this out myself before I posted. My newbie guess is either live bearer or molly "disease" (lack of electrolytes) or a swim bladder problem.
I am skeptical about livebearer disease because the tank is brackish and my water is hard.
I would greatly appreciate any ideas.
Thank you.

 

[OrionGirl]

Could it be parasites? Or a bacterial infection? Poor acclimation (not all LFS keep their mollies in BW--just a guess)?

BTW--true dwarf puffers are not brackish fish at all.

 

[cpr4cpu]

I checked with my LFS and they sell true brackish mollies (marine salt) and acclimated mollies (aquarium salt, not marine salt).

My question is how long has she been showing the signs of illness? Sometimes fish die of natural causes.. has she slowed/stopped eating? Does she have thin or frayed fins? There are just too many possibilities to make anthing other than a generalized guess.