yup, Leah is right. You are suficating your sand bed you will need to siphon your sand do it slowly to not have to much stuff flotting around your tank. You may want to do it a second time, but, before you do it the second time you will need to move the sand away from the walls of the tank. Using an algea scraper and scrape that bad black brown algea off the walls while you have a hose to siphon the bad stuff out of your tank. Then do a complete siphon of your sand again, do your water change and things should settle down from there.

Bill