I recently bought a house with a pool (first time pool owner). The edge of the pool seems to have staining that won't come up. I have tried brushing and and scrubbing it. It is only discolored on the ledge (The flat part around the pool where you can stand up no matter the depth). Does anyone have tips for this? It almost seems like the liner is stained. It has been like this since we opened the pool in April. I tried reading about it online and I *think* I need to buy a pool stain kit and, if so, does anyone have recommendations on brands for a beginner? Thank you in advance for any help. Sorry about the quality of photos but it is hard to show stains under water.