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Melanie
If there is a possibility to relocate the animals for about two weeks, then it's better to change all the sand at once without dragging out the process. It's also a good idea to stock up on pumps with sponges to siphon off the detritus. A couple of fresh stones wouldn't hurt to speed up the substrate maturation. I once changed river sand to aragonite - what a hassle, but what can you do?