a failure to thrive

[Scott S]

some time ago I bought three Firemouths and two Salvini. one of each have failed to grow or even seem happy in the tank, they eat, but their colors are weak and they never grow. the Firemouth eventually either died or was murdered, and now I worry the same fate awaits the Salvini. does anyone have any explanation as to why certain fish simply fail to thrive?
there was no evidence of bullying with the Firemouth, and none now with the Salvini, but the proof is there in the photos, the female is three times the size of the male and full of color, and they were the same size when I bought them.

the male:

the female:

 

[gilpi]

I've found this not to be uncommon. So many factors that can attribute stunted growth in a fish, from lack of water changes to improper feeding, tank size, temperature, tank mate compatibility and many more that can cause stress.
Since the other fish in the same tank are thriving, another possibility can be the fish history and what it went through before it got to your tank.

 

[HUKIT]

Genetics resulting from poor breeding techniques is the most common issue.

 

[meervanderjoop]

The male you think it's a male is a female.

 

[meervanderjoop]

And the other one is a male

 

[Scott S]

And the other one is a male

so the bright yellow one is male? I thought I read several places the black on the dorsal fin meant female.

 

[HUKIT]

And the other one is a male

That is a female salvini, the best methods of visually sexing salvini females is the black blotch on dorsal fin then followed by the presence of a black spot on the operculum.

Here is one of my pairs...