A properly functioning septic tank doesn't need enzymes. There are a few good practices that will give you a long life. First, don't use a garbage disposal. Food scraps should be composted or tossed in the garbage. The bacteria in the tank (they are naturally occuring bacteria) cannot effectively process the large chunks washed down by a disposal and they will either sing to the bottom and fill the tank faster or they will find their way into the leach field and clog that up.
next, go easy on harsh cleaners such as bleach. THese are great disinfectants and kill bacteria in your house and laundry, but they also kill the bacteria trying to work in the tank! A little bit wil dilute out and not cause a problem, get excessive and you will mess with the little ecosystem working hard at breaking down the mess.
Third, conserve water. You want to try to maintain at least a 24 hour holding time in the tank, though more is better. If you have a 1200 gallon septic tank, this means you want to flush less than 1200 gallons down the drain a day. Seems like a lot but it adds up fast. The longer holding time gives the tank time to process the effluent and not only maes it work more efficient but helps keep the leach feild healthy as well.
These three things alone will go a long ways towards keeping it functioning for a long, long time.
Brad