The Corydoras and the green/gold iridescent sheen

Hi,

I've had some Cory's for about 12 months now. They were sold to me as
Corydoras julii but as they've grown their markings have come to look
more like the reticulated form. Anyhow, I have recently been in the
process of upgrading one of my tanks and the two corys have been
temporarily housed in a 30 litre betta tank with my female betta and
two amano shrimp. Since moving them I've noticed that they have this
shimmering green iridescent sheen particularly around the gill covers
and on their flanks just behind their heads. The sheen extends in
increasingly tiny patches down to the tail and it quite beautiful.
However, under some light it looks golden which is what's causing me to
worry as I've read that Velvet can appear as tiny golden salt and
pepper spots. I've been keeping fish for a number of years and have
never had a case of velvet so I'm not really sure exactly what it looks
like. I can't see any bumps, spots or slime even under magnification -
just this sheen which seems to show through in irregular patches with
lots near the head and just a tiny smattering near the tail.

In terms of the fish themselves they seem to be perfectly normal - up
to all their normal cory antics - playing the the bubbles from the
filter return, actively scavanging for food in the substrate and plants
and going bananas if I give a feed of bloodworm or tubiflex! The betta
and the shrimp are all fine too.

My tank conditions are as follows:

Ammonia 0 ppm
Nitrite 0 ppm
Nitrate between 10 and 25 ppm (tap water is 20 around here)
Ph 7.6
Temp 79 F

The tank is fairly well planted and they get a weekly partial water
change with dechlorinated tap-water and a small squirt of plant min by
Tetra.

I'm thinking that this might just be their natural colours, perhaps
showing more now they are in a smaller tank and easier to observe but I
wondered what you all think - I don't want to miss it if it is a
problem.

There's a photo of the colouring I'm seeing at this location...


http://www.muddylittlefox.org.uk/corysheen.jpg

Thanks,

Muddy