09-20-2010, 03:24 PM
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Any mix of substrate types will "mix" with the smallest particles on the bottom (the sand in this case) and the gravel on top. A substrate is best if it is consistent throughout.
Either sand or small-grain gravel will work; if you want to go with one of the black enriched plant substrates, that's another option. Whichever you get, it should be 1-2 inches at the front and much deeper in the back, at least 3-4 inches if you intend Echinodorus (swords) that have extensive root systems and need the space. At this depth, sand can cause compaction problems that require extra effort to avoid, so bear that in mind.
I certainly agree that a dark colour, whether black, dark gray, brown or a "natural" mix is better, both for the fish and the appearance though the latter is a matter of personal choice. But the fish absolutely prefer a dark substrate as it is more natural to what they are evolved to live over. Being less stressed, they will show better colours over a dark substrate.