Originally Posted by EvilVOG
yes you can put the old gravel in a mesh bag or something to help with seeding.
If you take the existing substrate out in stages, it will transfer the load onto the filter and you won't have to worry about stalls. About a quarter of it every two days. Remember to HEAVILY siphon-sweep what you're removing, there are bacteria in it that will die when exposed to "high" light levels and they'll cause a spike.
I usually use a net to remove rock tho I have done 'live swaps' using a perforated ladel (like you'd use to lift out stew from broth) to lift out the large rock after the fluorite had been in with it for a month.
When you add in new substrate, use only about an inch thickness unless you're heavily planted. If you just have some plants in the corners then let them keep most their thickness.
The plants, if you have them, are the trick. Disturbing them can be bad for the tank. A plant is like a little sponge filter, so keep that in mind.