Most minor, single mouth ulcers are caused by damage to the mouth, for example by accidentally biting the inside of your cheek while eating, or from a sharp tooth, food or filling.
I have a 2005 Honda Element. The owners manual states the brake fluid should be replaced every 3 years. When I asked the dealer to replace the brake fluid, they said it was going to cost $190. I told them last time they did it, it was only $27.
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.