A calcium reactor is a device which is used to provide much needed calcium as well as other elements dependant upon the media used. They work by slowly passing water through the media and recirculating it via a small pump. CO2 is slowly applied to the device to reduce the pH level in the device so that the media slowly dissolves. When the media dissolved calcium is released which is slowly passed into the aquarium system.
But how do you know if and when you will actually need one?
Well realistically it depends upon what you are keeping? If you are keeping a lot of hard corals then quite probably before long you will need to add one. If you dont keep hard corals then you may be able to control your calcium levels by water changes, additives, kalkwasser etc.
The best thing to do is actually test your water. If you find that you cannot maintain your calcium levels by performing water changes then perhaps you should try additives. If you find that you are having to spend a lot of money on additives then the next logical step would be either try kalkwasser or go straight for a calcium reactor.
If you decide to purchase a calcium reactor then there are loads available on the market plus there are quite a few desigs available now for those DIY enthusiasts.
There are some people who purchase a calcium reactor at the very start just because they believe that they need one.
I don’t agree with this as the only way to tell if you actually need on is either by your test kits or by the amount of money you are spending on other methods. If you are spending hundreds over the year then to be honest it is time to invest in a calcium reactor.