I have a 125g tank with sand as substrate. Due to the messy nature of my oscar and my inability to vacuum the sand, I've decided to switch to gravel to improve water quality. My intention is to gradually (perhaps in the course of 4-6 days) remove the sand and eventually replace it with gravel. I don't want to take fish out but at the same time do not want to disturb their environment and/or bio-balance too much. Do you have any suggestions to make this transition relatively stress free for my fish?