Neptune Systems offers a range of modules and expansions that can be integrated into the Apex controller to expand its functionality and customization options. These modules include power bars, which allow users to control multiple devices such as pumps, heaters, and lights independently. Additionally, Neptune offers specialized expansions like the Trident, which provides automated water testing for key parameters like alkalinity, calcium, and magnesium, making it easier for users to maintain stable water chemistry in their aquariums.