The solenoid valves such as that in the picture, attached to a dispensing unit with CO2 cylinder, also rechargeable, have the task to open or close the flow of carbon dioxide, are in fact usually connected to a timer that regulates the ignition times and off or better yet to a pH meter. The electronic controller can precisely adjust the pH value because the law of a probe immersed in water and because holding open or close the flow of CO2.

Ensures water solenoid valve is used to
- Save the co2 at night, when the plants NOT need it, but ...
- To regulate carbon dioxide during the day
- Avoid, in the case of very soft water (low hardness, very low, near 0 °), that the pH may fall quickly or below a certain threshold for excess carbon dioxide (plants + cylinder)

In ' marine aquarium , however, the solenoid valve can also be used for other purposes, principally, however, is always used in association with a plant of CO2 and even more often in conjunction with a pH meter MA not to dispense CO2 in the aquarium, gasoline in the reactor of football.
The calcium reactor is a cylinder in which is dissolved material that releases calcium, which going into the aquarium water can be absorbed by marine organisms, first of all corals hard.

How to choose?
For freshwater aquariums is important to consider the purchase of the solenoid, because it may be unnecessary under conditions where the pH meter has not and does not take a particularly low KH. I are an example ... in practice I could just add it to save co2 ...
If you decide to buy it, attention to quality, because now even that is a product that is priced TOO much lower than 3-4 years ago, around 30-40 euros, but the quality is lower and therefore breakage more likely. Do not talk about security ... why still putting in a beautiful valley of the solenoid valve will prevent the water can end up in this attachment for the aquarium, we talk just broken, often burning the unit that should be bought back then ...
The problem is often the excessive heating (and therefore heat dissipation, waste of electric energy) that sooner or later leads to breakage. Un'accortezza may be to keep the solenoid valve in a position ventilated, or with a fan also slow before ... but it is not easy, so in all other cases recommend spending a little more (about 80-100 euros) for un'accessorio of higher quality sold by companies known and, if possible, with a low consumption of watts .
Do you think that looking online I found solenoid valves for aquarium use (because they also come for other uses ... then just fit) with variable consumption between 3.5 watts and 14 watts. Since labor is the same ... I really feel that one to 14 watts wasted as heat ... it increases the chances that you can burn.