This is often caused by one of two things: a lack of filtering (or a dirty filter) or a lack of shocking or boosting the chlorine level.
First: Make certain it’s not your filter. Check to make sure your getting good filter pressure by looking at the pressure gauge (located on the top of most filters). Good pressure is generally between 8 – 12 psi. If it’s higher than that, it could be the filter. A good rule of thumb: 8 – 12 psi is good, 12 – 16 psi is getting dirty, and anything past 20 psi means the system is dirty and likely needs to be cleaned. If you believe your filter is dirty, please move to the next question for instructions on cleaning. Another tip is to check how long your pool is scheduled to filter in a 24 hour period. Most filtration systems should run a minimum of 8 hours a day. You will never have a pool and spa technician argue that your running your filtration too long; the longer it runs the cleaner it will be.
Second: Consider a chlorine post. Most pools that rely on stabilized chlorine fed from the chlorinator will still need a good dose of shock (un-stabilized chlorine such as liquid or powdered chlorine) every 7 – 10 days. If you are unsure how much to add, a good ratio is generally one pound of powder or one gallon of liquid chlorine per every 10,000 gallons of pool water. If the water is especially cloudy, a double dose is recommended. If you are using a salt generation system, it likely has a button or location for boosting the chlorine output level. Be sure to only boost for 12 – 24 hours.