Best way to change substrates if you want to is to take all fish out and put them in a bucket, take all decorations/plants out (keep plants moist if possible, can float them in a bucket), drain the tank down to the gravel (make sure you got all the fish before you do this), and then do your substrate swap. Once new substrate is in, hardscape and plant it and then put your fish back in. If your water is super cloudy you may want to do one or two large water changes before putting your fish back in, but most of the time it should be ok unless you are working with a dirt tank (not recommended for a beginner).
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