Hello,
Interior dimensions: approx 23.6" W × 18.1" D × 13" H (600 × 460 × 330 mm).
18' circumference tank: An 18 ft circumference equals ~5.73 ft (68.8") diameter, which is way larger than the cover’s interior width. It will not fit.
Depth: The interior depth is 18.1".
Best regards,
VEVOR Team
By vevor