Is there a certain type of detergent you are suppose to put in the black tank? Is it laundry soap or dish soap? I'm reading all kinds of forums that tell you to put one cup of detergent with 1 gallon water down the toilet, and since my husband is away at work during the day, I try to learn what I can to share with him at night. Also, I heard that products with formaldehyde are bad for the tank. Is this true?