if u have live plants u will need 3 to 4 inches of gravel...if u dont about 2 or 3...3 being better...your gravel hosts bacteria which brake down all the food, poop, and maybe dead fish lying around lol...so if u dont have enough the effect doesnt work which makes the gravel a complete mess...how old is your tank...if its over 2 months old or so...u shouldnt have that much crude...after u clean the gravel...on the next gravel cleaning if its that bad again...i suggest more gravel...hope that helps