Welcome to the Amphilophus family
On the topic of picking the smallest, more than likely it will turn out female. Males from 3in really grow faster than females.
Before 3in in size
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After
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That black you see is the actively peeling areas. As fry these guys are silver and black hide me colors. Then they get that blotchy black look before changing to adult colors. Also if they get hurt as adults, the repairs come in as fry color and peels again.
As far as growth rate I can only comment on males growth rate as Casper did not let females live past 4in or so. I got 10in of size in the first year, so just under an inch a month or so.
Baby Casper
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6 months
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A year
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If it is a female, she should be able to to grow out to 12in, just takes her longer especially if she starts laying eggs. By her 5th spawn Casper had nearly trippled her size.
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