A kalk reactor is used to does kalkwasser to the tank, usually through an auto top-off setup. Kalkwasser is a high pH solution, usually made by mixing calcium hydroxide in fresh water, that helps replenish calcium and increase the pH of the tank. There are other benefits as well.

What kind of tank do you have? Sometimes there's no need to add stuff without reason.

 

all im going to have is a 55gal tank. But its not set up yet and wondering if i should get a kalk reactor or not. Keeping in mind that i think i can get a good deal on it.

 

it all depends on what you intend to keep in the 55 gal... if you plan on keeping softies... probably no, you won't need it, but LPS or SPS, you can use it... it's all about what you intend to keep