I just switched my 75 from gravel to sand. I emptied about 25 gallons into a rubbermaid tub and put the fish in that with a heater to keep the temp up. I scooped out all the gravel with a fish net. Let everything settle for a bit, then vacuumed up all the crap off the bottom glass. Then added sand, topped off the water, let that settle a bit, then put the fish back in! Easy! Most BB is in filter so I had no mini cycle... i'm also running three filters and have a small bioload so be sure to test still!