Dear customer,
The L-shaped/flat side of the O-ring faces UP toward the clear lid; the rounded side sits DOWN in the groove.
To fix the leak:
Lubricate the O-ring with pool-safe silicone lube.
Clean the groove and lid sealing surface completely.
Hand-tighten only—do not overtighten.
Replace the O-ring if it’s cracked, deformed, or dry.
By vevor