10-29-2009, 07:38 PM
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Previous posts have all been correct, because it depends upon the fish and what the setup is intended for. Plants and forest fish certainly look more colourful and settled over dark substrates (black, dark grey, dark brown) whereas Mitch's rift lake cichlids are used to lighter sand and rock. You should choose your substrate according to the type of aquarium aquascape you are aiming for.
I have personal experience in this; my former 70g SA tank had natural buff-coloured gravel. I moved the fish and plants into the 90g with the darker gravel. The Corydoras panda and similis darkened quite noticeably in the new setup, as did a group of Hyphessobrycon metae. The other interesting point is that the H. metae had been very secretive in the 70g, but from the first day in the 90g they are out and about. The reason I believe is that the darker substrate eases their anxiety. In the wild they are used to very dark and dimly-lit waters; the substrate is very dark sand, mud or leaves depending. Recreating this appearance as reasonably as we can will help to ensure their comfort, and that means they will be more visible and normal in behaviours.