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This can be very difficult unless the fish are full-grown. A. paucisqamis is smaller than A. mendezi. A. paucisquamis females show a hint of cross-banding in the caudal and usually show a black spot on the anus (anal spot). Their lateral spot is larger, horizontal & more oval than that of A. mendezi female. Like I said, it is hard unless you have several fish to compare with each other.
Mike is right that small mendezi and paucisquamis look very similar.
Anyhow, a couple of tips for you to distinguish them include:
1. For the two horizontal black bands across the body:
In mendezi, the upper one is much thicker than the lower one.
In paucisquamis, the difference is smaller.
2. Caudal fin pattern:
In paucisquamis, it is usually colorless and in some beautiful
specimen, the upper and lower tips on the caudal fin will be
a bit orange. Moreover, you could find some small dots in the caudal fin.
In mendezi, many specimen have orange/yellow markings near
the caudal base, and the caudual fin could have horizontal or
"big dot" pattern.