09-23-2011, 11:00 AM
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I have known corys to flop over on their sides. C. panda too if memory serves me correctly. It may be fine, or there may be trouble. I can't offer more, so just observe. Notice the respiration, if it is normal, probably nothing to worry about; if it is laboured or fast, may be something. Respiration will be faster after feeding, active swimming so that's normal; and slower if at rest. I usually try to compare the respiration of all fish in the tank if there is one fish that is questionable.
On the darkening, this is likely the fish. Fish have cells called melanophores that contain hundreds of melanin-filled pigment granules called melanosomes that can be dispersed throughout the individual cell (colour lightens) or congregated together (colour darkens). The cells are light sensitive and respond directly to light by what is termed melanosome translocations; this is why fish become pale and washed out during total darkness, and regain colour in daylight; this is non-voluntary. But fish can also voluntarily adjust these melanosomes in response to environmental stimuli, social interactions, and stress. Often fish will lighten their colour over white substrates (a sign of stress), and darken over darker substrates. I have seen this in my C. panda. I would here again carefully observe the group, along with other fish in the tank. Given the many reasons for this change, it could be an issue or it could simply be the fish responding naturally.