Huge Black Eye In Tropical Fish

[Kat888]

I bought a fish from walmart, is a live bearer, looks similar to a female swordfish. After introducing her to my small acquarium, about a week later, 2 fish (one fiddler crab one black molly died). She has one huge black eye, the other looks normal, and she is listless, hangs at the bottom, all fins held close to the body, kinda gasps for air on a regular basis, does have ONE tiny black spot on her small light-complected body, she had babies after I got her, but I think they all got eaten, I saw one, it also had a black spot on tail so I didn't think its a parasite. But now my two new fish (male and female swordfish) seem to be having same syptoms of listlesness and hang at the very bottom or very top of the acquarium. the two new were very active when I put them in but after only one day, they seem to be getting sick.

What is the deal with the bulging (but not bloated) black eye? Please help. Thank you for your response.

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Has the fish been kept with snails.
Black eye and black spots can also point to fish tb.
Does the fish look skinny or bloated.
Is the spine bending.
Any signs of flicking and rubbing.