Complex sentence: Redfin bullies of both sexes have distinctive diagonal stripes on the cheeks; these stripes are very useful for positive identification as they are visible in small (circa 30 mm TL) and very pale fish.
Split into simpler sentences: Redfin bullies of both sexes have distinctive diagonal stripes on the cheeks. These stripes are very useful for positive identification as they are visible in small (circa 30 mm TL) and very pale fish.