An aquarium photometer system includes a housing unit, an arm, and a mirror. The housing unit includes a light sensor configured to sense light incident on the light sensor and to convert the incident light to a signal. The housing unit also includes an operational amplifier including a first input node, a second input node, and an output node. The operational amplifier is configured to: receive the signal at the first input node, amplify a difference between the signal at the first input node and a signal at the second input node by a gain factor, and output the amplified signal on the output node. The housing unit also includes a potentiometer connected to the operational amplifier and configured to regulate the amplified signal; and a display connected to the potentiometer and configured to show an intensity of light detected by the light sensor based on the regulated amplified signal. The arm at a first end is connected to the housing unit and configured to move the housing unit around an aquarium case. The mirror is located on a bar and positioned within the aquarium in front of the light sensor and at a focal distance from the light sensor and configured to increase an amount of light incident on the light sensor.