Nothing. They both in the end perform that same task, adding calcium and alkalinity to the system in the correct ratio. It is just that they both do it in a different way.
Calcium reactor: uses carbon dioxide to dissolve a calcium carbonate substrate. Small amount of system water is flowed through it.
Nilsen reactor: uses calcium hydroxide dissolved in pure water. Some/all of the system evaporation top up water passes through it.
Thanks,I read the post on favorite top off systems and seen a few that said they used both on their system to counter balance each other and didn't understand.I thought they were the same.
They do have different effects on the system.
Calcium reactors add both carbonate and calcium to the system. They also tend to depress the pH a bit because of the co2.
Nielsen reactors add calcium and OH- to the system. They do not add carbonate hardness but the OH- ions can help preserve it. They also tend to raise the pH.
Using both in tandem can stabilize the pH and also add extra calcium over and above what one device could add.
.... in additon as well ..... using a Nilsen reactor does indirectly add alkalinity to the system, but it "generates" it from the CO2 dissolved in the water (actually it shifts the equilibrium point for the buffer system reactions by pushing it toward generating more hydrogen carbonate and carbonate).
