The add-ons available to the marine aquarist are un-ending. I'll start simple.
Wave makers. These control two or more pumps or power heads to provide a changing water flow to simulate the back and forth wave action found on natural reefs. They can go on to include systems with reservoir tanks where water is removed from and returned to the main tank to simulate high and low tides.
Lighting controllers. Sometimes no more than simple timers to give dawn and dusk effects by switching actinic blue lamps on an hour or so before the white lights and off an hour or so after. They can extend to systems controlling banks of lights to give simulated lunar cycles.
Dosers. These may be no more than a gravity run drip system adding water to the tank to replace evaporate. The replacement water may also contain trace elements like calcium and iodine. Larger systems may include a larger reservoir tank and a pump controlled by a float switch in the main tank to maintain levels.
Calcium reactors. These are very popular add-ons for reef tanks. Carbon dioxide is mixed with tank water to create an acid condition. This is then pumped through a column of aragonite or other calciferous material to increase the free calcium in the water. pH controllers can be used to regulate the activity of these systems.
Coolers. Reef tanks suffer if the temperature rises in hot weather. With all the electrics (pumps, lights etc.) tanks can overheat so coolers keep the temperature in bounds. Coolers, unlike heaters, are expensive but so are reef tanks and the livestock in them.
UV and Ozone are both used as sterilizers in marine tanks. Ultra violet lamps are used within a water jacket to destroy any bacteria or parasites free swimming in the water. They need a pump to pass the water through the tube but can also be fitted on the output side of a filter. Clean water allows the UV to penetrate the water effectively. Ozone (O3) is a three atom molecule of oxygen. Ozone is fairly unstable and the extra atom will attack proteins it comes into contact with. An electric arc is used to create ozone from a dry air supply. This can then be used in a protein skimmer to help get rid of excess protein in the water. Ozone can be troublesome. Even a low level in the air we breathe can cause headaches. The escaping air from the skimmer either needs to be vented outside or passed through active carbon to return it to the more stable O2 we normally breathe.
There are also computers available which will control all or most of the above.
Marine Fish and Invertebrate Systems Keeping : Berlin System : Tanks : Equipment Dreams : Marine Mud : Jaubert System :