My air plant developed strange concentric circles on the inside, and brown lines following the shape of the leaves. I had no issues for weeks until recently. The biggest change was when it was fertilised in 1-3 drops of Horti Orchid fertiliser in a large container filled with water (the normal weekly watering it gets). Could this be due to the fertiliser? It has been sitting under IKEA grow lights, which generate a lot of heat so I keep it far away from my plants.

Plant pictured is freshly watered.