most 'stick on glass' exterior thermometer strips are at most 5 to 6 inches long. when looking at the photo of the pleco beside the thermometer, it looks to not even be quite the full length of the thermometer, when taking into account the amount of thermometer visible and the length of fish visible.
not a foot long bn, nor an 8 inch. just a commonly sized 5 inch bn, maybe getting close to 6 inch. IMO.
fish are commonly measured for length using SL (standard length). this does not include the length of the tail fin. measurement is from tip of nose to where the tail fin connects to the body.