Although stable isotope analysis is a powerful tool for determining diet, migration patterns and the structure of food webs in aquatic systems, the slow response of isotopic ratios in the widely used muscle tissues of slow-growing fishes often hampers this approach. A diet-switch experiment was conducted to compare changes in δ¹³C values in epidermal mucus and muscle tissue, using 5-year-old catfish and young-of-the-year crucian carp. Isotope ratios in mucus, measurements that can be non-invasive, changed rapidly whereas muscle tissue did not change obviously in both species examined. As isotopic change in mucus occurs quickly, this technique could be used over a finer time scale than traditional isotopic analyses using muscle tissue. Using mucus alongside muscle would enable a multiple tissue approach to determine food habits of slow-growing fishes over different time scales. Larger fish had a slower rate of isotopic change compared with smaller individuals, in both species. The half-life of isotopic change in mucus was 141 and 78 days in the catfish and the crucian carp, respectively. As body size and species influence turnover and discrimination rates, these rates need to be understood prior to field data interpretation.