Yellow lower leaves on a peace lily: the watering pattern gives a clue

Your peace lily looks basically well, but yellow leaves at the bottom are typical of a plant allowed to dry out too much between waterings. You say it is watered every fortnight or less often, which is frequently too little for a peace lily, especially in a bright window. The fact that mainly the older, lower leaves are yellowing fits that too.

Cut completely yellow leaves off at the base. Then water thoroughly so that the whole rootball is moist, but do not leave the pot standing in water. From now on, feel the compost and water when the top two to three centimetres have dried out rather than sticking to a long interval.

As it is September and light levels are falling, it may need a little less water than in midsummer, but still needs consistent moisture. If new leaves keep yellowing despite more even watering, lift out the inner pot and check the roots: dark, soft roots suggest it has sat too wet at some point, while a hard, very dry rootball confirms drying out. Adjust the watering now and watch whether new leaves stay green over the next two to three weeks.