Hi Shunkun, the method you've said seems ok. To speed up the process, you can use fine net to get the top soil from your gravel (also contains beneficial bacteria) and place it on your new gravel. I would say your friend is also right. Your water parameters might change after the change of soil. So let the tank restabilise for about 3 weeks or so. And don't dump all the fish back immediately. Give a gap of about 1 week or so for every group of fish.
You would also have to prepare the sub tank. Start by getting a simpler filter to mature by letting it run in your main tank (along with your original filter) for a while. Use some of your old filter medium in your new filter as well. And, again, use some topsoil in your sub tank. Hope this helps.
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