In experiment 1, five goldfish (Carassius auratus) paddle-pressed on fixed-interval (FI) and variable-interval (VI) schedules for food pellet reinforcement. The order of conditions was FI 60 s, FI 240 s, FI 30 s, FI 60 s, and VI 60 s. FI responding showed a scalloped pattern and response-rate break points were proportional to interval duration. Post-food wait times varied with interval duration, but were not proportional. Response rate on VI was constant. Experiment 2 studied the properties of food reinforcement as a time marker. The same five fish were presented an FI 60 s schedule of reinforcement with 25% of intervals ending in non-reinforcement (N). The fish responded faster and paused less following the omission stimulus (omission effect) and response rate was flat or declined through post-N intervals.