12-14-2012, 10:36 AM
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Possibly, yes, as you would be suddenly suffocating the existing substrate. It is never good to mix different substrates, thinking here of different sizes, as Boredomb mentioned; they will mix with the gravel on top.
The substrate is the most obvious part of the aquascape if it is "different" in some way, and as it is also the most complicated to replace it is best to decide what you want and do it at the start. I have changed my tank substrates over many times over the years, and during the past 2 years five of seven tanks were torn down and changed. It is the easiest way to do it.