I clean septic tanks on average every 1-3 years. Different climates affect septic tanks differently. My father had a tank years ago in Florida, that needed cleaning less often, possibly because temperature of system is more constant, where tanks in New England slow down in winter when tempertaure drops. I find bacterial flies and worms in septic tanks in summer, but never in winter.People who have health problems or take medications usually have a sick septic tank or system. These tanks will smell worse and won't have the proper seperation as a healthy tank. Most newer systems have filters and that is a large part of my business now as an extra service I do at time of pumping or on service calls. Well water that has minerals, iron, magenese will cause extra sludge in tank,especially softener discharge, which is harmfull to newer systems. Depending where you live, it may be legal or illegal to drain softener discharge elsewhere. plowking