A calcium reactor is a chamber filled with crushed shells/aragonite material under continous flow that dissolves this material with the use of carbon dioxide.
Water is circulated through the container, flowing from the bottom up to prevent clogging. CO 2 is injected into the water in the reactor and lowers the pH, this lowering of the pH, dissolves the aragonite material into your water column, and from the reactor it slowly drips into your tank. A calcium reactor will raise your calcium and alkalinity. At first it will lower your pH but eventually when your alkalinity is high enough it won't have as big as an effect on your pH.
The pH change is the reason your hear many people talking about running kalkwasser reactors, they raise the pH back up and are usually run in team with calcium reactors at first, but are not always a necessity.
The first week of dialing in a reactor is a little tough, but once they are dialed in, they work great and are for the most part maintenance free except for every couple months. 46gal is a bit small, so your choices may be limited in what you can buy, but there are some out there still that will fit that tank. Be ready to spend a few hundred bucks getting it going.
If I had to recommend anything to you, why don't you run a 5 gal auto topoff on a float switch mixed with kalkwasser and water and try that for a couple months before jumping into a calcium reactor.
