Let me evaluate it from the perspective of a customer. In the gym I have been to at present, bathing is free. If you charge, you will be reviled by guests, and it is also possible that some customers will no longer choose your gym. It is best not to add water control system. You can add a high-power non-solar water heater as a backup (after all, you can't always run out of hot water). A water-saving note is posted in the bathroom, indicating the limit of hot water. If there is really no hot water one day, customers will understand. As for the issue of sharing money between the two sides, it is not convenient for outsiders to intervene. I hope you can understand each other and maintain the original good relationship.