All I have to say is you get what you pay for or if you pay too little, you get what you don’t want to pay for as well.
Be honest with yourself and your employees. If you are paying well, you can expect a little more effort from the trainers. Say so up front, as in “I am paying you 25% more than the area market. That is because I expect everyone to pitch in on keeping the gym clean and tidy. And always be professional”. Put it in writing what the pay is and what the trainer’s duties are going to be.
There are a lot of little and big perks that you can bring in later if things go well. Like in house CEC programs and bonuses.