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Newly imported suckermouths, particularly Peckoltia, are often much more yellow/orange than I can keep them in my tanks. I assume this is a dietary issue.
Does anyone else notice this? Any ideas what this ?coloring agent? might be?

Kevin Korotev
Milwaukee, WI USA

Kevin,
If you think this is the case with loricariids, it is 500% more notable with corys. I think there are several issues:
1) Diet; As you pointed out. A very caring aquarist might feed 4-5 types of food with regularity. Most captive fishes likely get 1 or 2 (say tablets and frozen bloodworms). This just is not the same as a wild fish eating dozns of spp of insects, algaes, freshwater sponges, etc, etc.
2) Water; Very few people mimic whitewaters and blackwaters. Most people aim for a crystal clear tank. I can say from experience that clearwaters always have the least colorful fishes. Colors seem to come out more when a fish has to "advertise" due to poor visibility. I have seen fishes color up again when placed in a proper blackwater tank.
3) Sunlight; Anyone that has kept some tropicals outside in the summer knows that natural sunlight causes fishes to color up.

-Shane

I agree that water quality is an important factor for their colouration, but I maybe not agree to 100% that more turbid water have more colourful species... Rio Tocantins, Rio Xingu and Rio Tapajos are all clearwater rivers even if the last mentioned can be quite turbid caused of the goldmining.
Many of the colourful species lives deep in the river where not so much sunlight reach, very dark so the divers that collect the fishes must use flashlights.
So, the conclusion maybe could be... the darker or turbid water the more contrasted colour is needed to be seen (not all fishes want to be seen and they are usual grey or silver).

When loricariids and corys arrive after a shipment, allmost all look like they lost their colours. After some days in water with salt and high oxygen level, they start to get very pretty colours. The same in our home aquariums, same day or the day after our weekly waterchanges they get more pretty and sharper in their colouration...and active. A tank that have been running for some time can have very poor water quality most of the time even that we do weekly water changes (except directly after a bigger water change) but between the water changes it's very poor quality, the biomass can be very high... and that includes not only fishes, bacteria and all the organic waste is the biggest problem.
You can do an experiment, set up a new tank from scratch and move your fishes when it's cycled...your fishes will get their pretty colouration back.

Then of course other factors like temp and food also affect their colouration.

Janne

This is obviously a very complex subject and I agree with janne and shane. This subject has an interesting parallel with the poison dart frogs. Wild specimens are very toxic and captive bred are non-toxic.

All we can try is our best but it is ultimately impossible to duplicate the native biotope/diets our fishes have evolved in.