What's the best way to change the gravel in your tank? Total tank teardown? If so, I'll lose all my bacteria in the gravel. OR, should I change 1/4 of the tank at a time (maybe 1/4 of the tank per week) Would this work? This way the original bacteria can grow on the new sand. Will the P's be bothered by the changing gravel colors? BTW- wanna go from black gravel to sand (big color difference). Any suggestions?