The sms122 ph controller by milwaukee makes it so you can easily monitor and maintain a steady ph in any given body of water. 

USE COUPON CODE SMS122 COUPON FOR A SPECIAL DISCOUNT this item only

What it does is when used in conjunction of a co2 regulator like the ma957 that has a solenoid it facilitates the on off cycles  with the solenoid.

Why would you want that you ask.  Well using co2 in an aquarium causes a lowering of ph.  If the ph gets to low this can cause stress on the other occupants in the tank including plants, corals, and fish.

So you use an sms122 co2 - ph controller to make a set point.

Say you want your ph to stay @ 7.0  You make the set point 7.0 and the co2 stays running from the regulator such as the Ma957 with a solenoid valve until 7.0 is reached.  Once the ph goes up from no co2 being pumped into the aquarium then the unit turns back on.    The SMS122  will create a more stable enviroment for your tank.