What is causing the yellowing or staining of a liner?

Yellowing or staining of a liner could be caused by several factors:

1. Chemical Reaction: If the water's chemical balance is off, particularly with regard to chlorine or pH levels, it could cause staining. An imbalance might react with the liner's material or create conditions that cause other staining substances to form.

2. Metallic Stains: Metals like iron, copper, or manganese in the water can create stains. These metals might be introduced through the source water, or they might leach from metal components of the pool or pond system.

3. Organic Material: Organic matter such as leaves, algae, or other plant material can decompose and leave tannins or other staining substances behind.

4. Sunlight Exposure: UV rays from sunlight can cause discoloration of the liner over time, leading to a yellowing effect.

5. Use of Inappropriate Chemicals: Utilizing cleaning agents or treatments that aren't compatible with the liner's material could cause discoloration or damage.

6. Water Quality Issues: High mineral content or other contaminants in the water can lead to staining.

7. Aging of the Liner: Over time, the material of the liner may naturally degrade and discolor.

8. Poor Water Circulation: Stagnant areas might allow substances that can stain the liner to build up.

Determining the exact cause can sometimes be complex, and may require testing the water and closely examining the stain. Solutions might include adjusting the water's chemical balance, using a stain remover designed for the specific type of stain, replacing damaged components that might be leaching metals, or increasing water circulation. In severe cases, replacing the liner might be necessary.