In 30 years of fish-keeping I have never seen this.
Fish have a preference for substrate color?
Ill tell that to my wild discus and wild eartheaters who are on white ( now slightly tan) sand.
This color is all wrong for you- lol!
It is white substrate that is the problem. This does affect many fish because they do not expect bright but dark substrates. It becomes even worse under the tank lighting which is usually far brighter than what the fish experience in nature as they tend to come from forest watercourses that are often well shaded.
The dark substrate is why fish are darker on the dorsal side and much lighter on the underside. Over their habitat substrates their darker dorsal blends in more with the substrate so they are less likely to be preyed upon, while from underneath the lighter underside is more difficult to see against the surface, and predation is less likely.
Some fish will also pale in colouration depending upon the substrate. This is why the ichthyologists and biologists always refer to "dark substrate." However, one can go too far that way too with some species. Ian Fuller was commenting on black being unsuitable for Corydoras and noting their change of colour because of it, and that such things are stressful.
I have had proof of thee effect of the substrate on the colouration of some of my fish when moving them from a darker to a lighter substrate (though never white or black) or the reverse.
If you have ever observed fish in their habitat, or seen videos of the same, it is amazing how species like
Corydoras duplicareus for example are absolutely invisible over the sand which is the same colour as their body colouration, and their black dorsal stripe matches the branches littering their habitat substrate. When they move it becomes obvious they are there, but nature has provided fish with some well-thought out defenses and we should take note and provide similar if we want healthy fish.
