its very rare when u sun bleach a plant an highly unlikely just from sun though a window, my tank gets full sun all day id look into if it is being eatin by the pleco or another fish or a nutrient deficiency ( are u using any ferts). what type of light is it? watts varys depending on the fixture so they type of light then wattage helps give a much better assessment, 15w might be to low of light but with the sun light might have enough to sustain it, are there any other plants if so what kind and are they doing ok or having issues to? is it on all leaves or starts at the bottom of the plant? all these little things can help give much better advice on what is likely your issue. so any of that info will help an list your fauna as they could be the cause to, hope it helps ya get it figured out.