08-10-2009, 10:15 AM
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Assuming everything's OK as 1077 mentioned (no water problems), then FishinPole has it. Many fish, including angels, change their colour or pattern during darkness. Some fish more than others. Pencilfish for instance completely change...you would not recognize them as the same fish; all the lateral lines on all species become separated dots during darkness. Angels also do this (the striped ones).
I'm not a biologist and have never researched why this is, but it is completely natural. I have on a couple of occassions checked my tank well after darkness with a flashlight. In case you're wondering why, I did this a couple of times after first acquiring a trio of driftwood cats (Tatia perugiae) that spend all daylight hours sleeping in tunnels in the bogwood (hence their common name). They only become active and feed at night. I never saw them and wanted to ensure they were OK. I found them, swimming around playfully, eating tablet food put in for them; but I also noticed all the characins were greyish, faded colour and little pattern. I knew this occurred, just never had reason to see it before.