Visual determination of malachite green in sea fish samples

•The visual determination of malachite green in fish.•A simple visual method for use in the field.•The solid phase extraction of malachite green in polymer matrix. Polymethylmethacrylate matrix has been developed as an analytical device for visual and spectrophotometric determination of malachite green. Based on the studied effects of different pH levels, temperature, contact time and diverse ions, optimum analysis conditions have been established. For malachite green, the calibration graph was linear in the range of 0.5–30 mg·L−1 in the samples with the detection limit of 1 mg·kg−1. The proposed simple method makes it possible to determine malachite green visually in the concentration range 8–30 mg·L−1 using a colour scale. The method has been implemented for determination of malachite green in various sea fish samples.