i am planning to add new sand in with what i aready have and since im doing this i wand to move some rocks around, is it safe to do this with fish in the tank? i was told to use a pvc pipe when adding the sand and im not sure if i should let it settle before moving rocks around or should i put most of the rocks to one side and do the tank by parts? are there any risks doing this? anyone with some good tips? thanks