Lyme disease is diagnosed clinically based on symptoms, objective physical findings (such as EM, facial palsy, or arthritis), or a history of possible exposure to infected ticks, as well as serological blood tests. The EM rash is not always a bull's eye, i.e., it can be solid red.
Rule of Thumb for Changing Filters. The rule we have followed for 18 years in the filtration industry is to change filters every 6 to 12 months. Six months is almost always optimum, especially in commercial/office settings, and twelve months is as long as you want to go no matter what.
Once you determine a schedule that works for you, testing can be done less frequently to confirm your routine is adequate. As a general guideline, you could start with a 20-30% water change once a week. Some people prefer to do a larger change less frequently, such as 40-50% every two weeks. I believe the latter is less desirable since the water chemistry changes more dramatically at once, putting more stress on the fish.