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Younger A. mendezi can be difficult to sex. In general the tips of the soft parts of dorsal, anal and ventral fins should be less pointed/elongated as in males, females should show less pronounced pattern in the caudal fin and the metallic bluish markings on the head should be less pronounced...
I have very strong doubts that this fish is from the Orinoco let alone from the Orinoco delta!
I have added a bit of contrast to the picture:
Of course, it is not possible to be 100% sure from such a picture but my guess is that your fish is A. norberti.
The features that make me think so...
Your fish are either a form of A. cf. cacatuoides or A. juruensis. However, it seems that your male has a small orange spot near the upper edge of the caudal fin. If so, it's A. cf. cacatuoides. Do you have any information on the catch location?
Most noticeably on the head.
I never kept wild nijsseni-group fish,. so I don't know what happens to the red in their caudal fins.
If it was subadult when you got it, it's color will of course increase while maturing. The question is, will it become as colorful as it would have been in the wild?
Yes, my wild Apistos did get a good diet but the red coloration nevertheless did fade.
Of course, fish from a shop are usually kept in "less than perfect" conditions and therefore don't show all their colors unless the conditions improve.
My experience with wild caught Apistos is that over time particularly the red coloration fades more and more. I didn't found a way to stop or even reverse this.