Q.avocado plant in a pot
Leaves are turning brittle on the sides and ends.
I thought it was perhaps watering with a plant food so I stopped that and am using just water. Still happening, in fact, even new growths come out with the black/brown dry edges on the leaves.
You were likely over fertilizing the plant. An application of fertilizer once a month is recommended.
The excess fertilizer will build up in the soil and the leaves will turn brown or even black. The salts from watering with conditioned water will also cause this.
Distilled water can be used to help prevent this.
You can flush the soil with water to try to remove some of the build up.
The soil should dry between waterings.
Here is a link with more information.