According to the government statement (for details: news.gov.hk), the government introduced two new “vaccine bubble” arrangements for fitness centers. Under the condition that the fitness centres have complied with the requirements such as ventilation standards, the maximum number of participants in each sub-group of training groups or classes will be relaxed to 12 persons provided that all staff members have completed a vaccination course and all participants are wearing masks, and customers will be allowed to exercise without wearing a mask if all staff members and users have completed a vaccination course.

 Two new “vaccine bubble” arrangements for fitness centres will be introduced:
(a) Training groups or classes comprising sub-groups of not more than 12 persons are allowed subject to the following conditions:
(i) all staff members present in the room where the training or classes are conducted must have completed the vaccination course, and those who are unfit to receive vaccination because of health reasons must undergo regular testing;
(ii) participants are arranged in sub-groups of no more than 12 persons and a distance of at least 1.5 metres between each sub-group is maintained or some form of partition is in place;
(iii) all persons who are present in the room must wear a mask at all times (including when exercising);
(iv) in the course of the training or conducting a class, the coach must stay at a fixed location at least 1.5 metres away from any other person in the room; and
(v) in respect of the room concerned, the air change per hour has reached six or above, or air purifiers that meet the specified specifications have been installed, and a certificate thereon issued by a registered specialist contractor is displayed at a conspicuous location of the premises.

(b) customers are allowed to exercise without wearing masks, subject to the compliance of the following requirements in the fitness centre:
(i) all staff members and customers must have completed the vaccination course, and staff members who are unfit to receive vaccination because of health reasons must undergo regular testing and wear masks;
(ii) the air change per hour has reached six or above or air purifiers that meet the specified specifications have been installed in all exercise areas, and a certificate thereon issued by a registered specialist contractor is displayed at a conspicuous location of the premises; and
(iii) all users must scan the “LeaveHomeSafe” venue QR code using the “LeaveHomeSafe” mobile application on his or her mobile phone before entering the fitness centre.

You may check the air purifiers FEHD approved here: https://www.fehd.gov.hk/english/licensing/guide_general_reference/Information_air-changes_purification.html