There is a wealth of game theoretical approaches to the evolution and maintenance of cooperation between unrelated individuals and accumulating empirical tests of these models. This contrasts strongly with our lack of knowledge on proximate causes of cooperative behaviour. Marine cleaning mutualism has been used as a model system to address functional aspects of conflict resolution: client reef fish benefit from cleaning interactions through parasite removal, but cleaner fish Labroides dimidiatus prefer client mucus. Hence, feeding against their preference represents cooperative behaviour in cleaners. Cleaners regularly cheat non-predatory clients while they rarely cheat predatory clients. Here, we asked how precisely cleaners can adjust service quality from one interaction to the next. We found that non-predatory clients receive a better service if the previous client was a predator than if the previous client was a non-predator. In a related laboratory experiment, a hand-net used as a stressor resulted in cleaners feeding more against their preference in subsequent interactions. The combination of the cleaners’ behaviour in the two studies shows that the cleaners’ service quality for a given client species is not fixed, but it can be manipulated. The results suggest that short-term stress is one factor that causes cleaners to increase their levels of cooperation, a hypothesis that is amenable to further experiments manipulating the endocrine system.