BEN Dunne's Westpoint Gym lost 500 members after a controversial decision to pull swimming and other water facilities at the club.
Dunne said he made a mistake and reversed the decision.
Although he was reluctant to comment on the exact number of members who cancelled their membership, Dunne said: "If we continued on that track we could have lost up to 25% of our members at Westpoint".
The gym has around 8,000 members.
"It was a very bad move because you can't turn a jumbo jet into a 737," he said. "It was a bad decision."
The group could have lost members at other clubs had the same mistake been made, he said, but added that the decision to remove the pool was limited to Westpoint.
"People [in the Carlisle and Northwood gyms] did say that if they took out the pools there, they would move," he said.
Dunne said the reason for his decision to remove the water facilities was a plan to mimic the successful strategy of the successful European gym, McFit.
"They were pointed out to me by my son and I suppose in enthusiasm to get one of them going immediately I decided to close the swimming pool area in our gym," he said.
"Thankfully with the good people around me it was pointed out to be a bad decision I made.
"We wanted to test the formula in one gym; going forward most of the new gyms will be opened without pools but not all of them."
The former Dunnes boss said when he eventually opens his new gyms based on the McFit operation, they will have a separate brand name to the three gyms already up and running.
His team is in discussions on three potential sites for the new enterprise, although given the instability of the property market, Dunne is uncertain as to when, where or how many of the new clubs he will open.