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Can some one explain why some Cory catfish of the same type (Albino Corydoras aeneus, for instance) vary so much in the iridescence? Is is age or diet? Male vs Female? I'm assuming an individual's genetics has a big part, but are there environmental reasons also?

I would imagine its largely environmental conditions like water clarity, lights, and how the fish swims and its overall shape all come into play- I'm not sure if I would assume its genetic or that I would assume anything for that matter. If it is a polygenic trait then you would be able to produce some interesting fish a few years down the road (in theory).

I don't know about diet but sex would certainly affect the over all shape of the fish and therefor the light would bounce off in a different way.

Yes, I hadn't thought of it in that way. The shape of the fish would effect the way the light bounces off the armor plates.

The reason why I'm pondering such things is that I'm watching 4 young albino(bronze) corys "color up" in the QTank. Watching this happen in albinos (without pigment to color up) has been interesting. These corys were from a lfs. With decent food they have gone from a translucent pink with thin white outlines of the body armor plates, to the body plates becoming opaque and for some of them,a lovely iridescent. I'm just curious. Surely some cory lover has manipulated diet or environment to accentuate this iridescence ? Or is it really only a matter of genetic lottery alone? Thanks All

Iridescence in animals is normally a structural effect, not a pigment. This means that the microstructure of the tissue creates the color. In colors produced by pigments diet can change the color by providing more or less pigment. In structural colors the color is more or less set once the tissue is made, although there are probably exceptions to this. This limits what can alter these colors.

Also substrate color can affect iridescence. The iridophores (pigment cells which create the shiny color) can be masked and unmasked by melanophores (black/dark colors) depending on the environmental background and the mood of the fish.

Cheers, Eric