EA environment manager Gordon Whitaker said the incident was “serious and avoidable”, and caused by the “negligence of both parties”.
Both companies were also ordered to pay a total of £45,262 in costs following a prosecution by the Environment Agency.
KMI Plus was contracted by UU to carry out improvement works at Wayoh water treatment works at Turton Bottoms.
In December 2013, as part of the works, the company emptied and removed a tank which had been used to store sodium hypochlorite in 10 per cent solution.
The majority of the contents of the tank was removed, but up to 300 litres of the highly toxic chemical was left in the bottom and needed to be emptied.
Sodium hypochlorite is used in the water purification process and is also the principle ingredient of household bleach. It is very corrosive and is highly toxic to aquatic organisms.
Counsel Richard Bradley, prosecuting on behalf of the EA, told the Court that instead of pumping the remaining liquid out, a decision was taken to put a hosepipe into the tank and dilute the sodium hypochlorite with water.
To do this, KMI let it overflow into a bunded area, and left the hosepipe running unattended for 15 hours.
Bradley said this was carried out without any risk assessment or method statement, and the companies “were not in agreement” on what they understood had been agreed prior to the removal taking place.
Up to 900 dead fish were recovered, including Brown Trout, Loaches and Bullheads, but the number killed is likely to be much greater.
At the end of June 2014 the brook had recovered sufficiently for restocking to take place, and UU paid for the brook to be restocked with native fish from the downstream section.
Judge Timothy Clayson said the incident had arisen through senior management failings to ensure proper system and procedures were in place.
Both UU and KMI pleaded guilty at the first available opportunity and cooperated with the prosecution.
A UU spokesman said: “We fully accept the court’s decision in light of the regrettable environmental impact caused by this incident.
“We take our environmental responsibility very seriously and have since reviewed our process and procedures to minimise the risk of an incident like this happening in the future.”