Brighton Crown Court has fined Solvent Resource Management £150,000 over the incident at Rye in 2009 which saw a large quantity of waste solvent and water released on to the surrounding land at a chemical plant.
The incident, in March 2009, saw a steel tank weakened by internal corrosion collapse on to a retaining wall and release around 340t of solvent and contaminated waste water into the surrounding area.
The chemicals were potentially hazardous and highly inflammable.
A valve was knocked off an adjoining tank by the incident, so releasing a further 90t of contaminated waste water on to the site and surrounding property.
East Sussex Fire and Rescue Service placed a 300m cordon around the site for two days.
Investigation by the Health & Safety Executive and Environment Agency revealed a failure to manage examination and inspection of the site's tanks and a prohibition notice was issued until the others had been inspected.
A separate improvement notice was also issued.
Solvent Resource Management of Hanson House, Castle Hill, Maidenhead, pleaded guilty to breaching Regulation 4 of the Control of Major Accident Hazards Regulations 1999.
The company was fined £150,000 with costs of £20,000.
"It was only timing that prevented this incident being more than a significant disruption to local residents and businesses," said HSE inspector Trevor Jones.
"If the company had put in place suitable and effective measures to manage the tank inspection programme, according to its contents and use, then this incident would have been prevented."
