B&Q admitted breaching the health and safety laws by exposing people at its Longwell Green store, near Bristol, to the risk of Legionnaires' disease.
North Avon magistrates fined B&Q the maximum amount on Tuesday.
South Gloucestershire Council said more than 500,000 people could have been exposed to the disease.
But it acknowledged that there had been only two cases of Legionnaires' in the district since 2005.
A B&Q spokeswoman said: "The breach occurred when the Longwell Green store failed to follow company process by filling a display hot tub with water.
"This contravenes company policy. When B&Q was made aware of the situation all stores were re-issued with a mandatory notice banning the display of working hot tubs.
"B&Q would like to assure its customers that the health and safety of its customers and employees is of the utmost importance. We are not aware of any sickness reported by customers or employees as a result of this breach."
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