My understanding is that there are two different types of product involved - a 'starter' pack and a 'maintainance' pack. They can be either liquid or powder. The 'starter' is for use in a new tank, one that has been emptied or one where the bacteria have died. Bacteria can die if the tank dries out through lack of use, nasty chemicals or (worth remembering) if anyone in the house takes a long or high dose course of antibiotics. The 'maintainance' is for weekly or monthly use (follow the instructions on the pack) and acts as a booster for the bacteria. The happier and healthier the bacteria, the less chance you have of getting nasty smells! I'm not an expert, by any means, but I chuck my wee pack of Eparcyl down the loo every week and it seems very happy - well, as happy as a tank full of sh** can ever be!! [:)]