Do NOT use a household sponge, even if it is new out of the package. They have chemicals on them to kill bacteria, so the sponge doesn't get nasty, being that they are almost always damp when being used in the house. Only use sponges, scrapers, scrubbers, suitable for aquarium use. You can use white vinegar to break up the residue in the tank. Scrub the tank out then rinse with water.