ring

Woke up this morning to check on my Oscar and noticed an oval ring on her side. I honestly have no idea what it is… Tank is 5ftX1.5ftX1.5ft Temp: 27.5°c Ph: 6-6.5 Nitrites and ammonia: 0ppm Nitrates: 25ppm (about to do a water change) Filter: 1800Lp/h other tank inhabitants: Senegal bichir...