I agree with Sue D’Alonzo. If you have an instructor that packs in the students, give them some type of per student pay out. Or quarterly bonus. And don’t forget to express your appreciation of their hard work.
For less popular instructors you should still give them a pay increase that is not too small to be appreciated. Come up with an amount that you feel would be reasonable for the year and divide it by their annual average hours to increase their hourly rate by. Be sure to thank them for their hard work as well and point out the annual dollar amount that their raise will translate into for them.
I hope your instructors appreciate their increases. Many fitness businesses never give raises and have a high turnover as a result. And I hope appreciate loyal instructors, even the ones that are “difficult”. They make your life easier than hiring new people over and over.