Scandinavian Design Ltd of Aberbargoed, in Caerphilly county, was fined £8,200 and told to pay £9,000 costs after admitting two pollution charges.
About 100 trout died when solvents entered the River Rhymney after a fire at the gift wrapping firm in May 2005.
Magistrates were told that the company had paid £200,000 in clean-up costs.
Two ponds were also polluted, which have since been dredged and relined.
The company has also invested £1.2m to upgrade the drainage infrastructure and also storage facilities for the chemicals kept at the site.
After the case, the agency's environment officer Eleanor Smart said: "It is important to remember that even in the event of an emergency, causing pollution is still liable to enforcement action.
"It is essential that businesses take all the necessary precautions to ensure that both wastes and materials are stored safely and contingency plans drawn up."
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