Fire service group manager David Diggins said crews were called to reports of people who had been “overcome by the chemical” and were feeling nauseous.
He told the there had been “a mixture of some chemicals within the systems” of the pool, which had been overfilled.
“There is no wider risk of chemicals in the area and the incident has now been fully resolved,” he added.
A spokesperson for East Midlands Ambulance Service (EMAS) said it sent “a number of resources” after receiving a call reporting a medical emergency.
EMAS said it took four patients to the Queen’s Medical Centre in Nottingham.
The Health and Safety Executive (HSE) has been informed, and the manager on duty told the the gym remained open.
