Calibration fluids are specialised solutions used to calibrate and verify the accuracy of measuring instruments and sensors in the aquarium hobby, such as pH probes and refractometers. These fluids are formulated to have known and precise values for specific parameters, such as pH or salinity. By using calibration fluids, aquarium enthusiasts can ensure that their monitoring equipment provides accurate and reliable readings, leading to better control and maintenance of the aquarium's water quality.