i have a newtank and i set it up a while ago the levels are good and i want to change the subsrtate soon. right now I have marbles as the substrate in a 5.5 gallon tank. I have been told to keep some of the old marbles in a nylon bg to kep some of the bacteria . now here's the question can i go about it a diferent way and put some of the new cleaned gravel in a nylon bag to collect bacteria ? and then change it all in a couple weeks?