Substrate

[t. smith]

I'm have to empty my 300g tank to fix some problems. When I pull all my substrate out can I reuse it or do I need to replace it.

 

[Tristan]

If you're live rock will still be live when you put your tank back together I recommend just rinsing all the sand with tap water because of all the trapped detritus, fish poop, and all that other nasty stuff.
In time the sand bed will repopulate with all those critters. Ex. Worms. If your live rock is really good(old/ has a lot of bio diversity) should repopulate the sand bed quite quickly.