11-21-2012, 12:28 PM
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All the suggestions from others are pertinent. Water parameters (GH especially, also pH, and temperature), the physical environment [both species prefer thickly-planted tanks, or alternatively tanks thick with branches and floating plants], minimal light, minimal water movement, a dark substrate, and tankmates. Food plays an important role too, but with the highly-nutritional prepared foods available today, this is not the aspect most likely to affect colour.
Neon tetra are not as colourful now as the cardinal, far from it. The years of inbreeding the commercially raised neons are caused considerable weakening of this fish. The cardinal in many areas are still mostly wild caught, though tank-raised fish are becoming more common in some parts.