04-24-2013, 11:59 AM
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I agree on doing the change from gravel to sand now. The longer the gravel stays, the more bacteria will colonize it, and when you change the substrate it will be even more of a change biologically. And I am not just considering nitrification bacteria, which frankly is the least of the issues, but all the other types of bacteria that live in the substrate and keep an aquarium healthy.
I also do not recommend changing a bit at a time; do it all together. I have changed the substrates in all my tanks more than once in some cases, from a 10g up to 115g. Live plants, especially floating, also benefit.