Dosers and Calcium Reactors are very useful in reef systems. They provide calcium and other nutrients to your water when they are low and needed. They are a necessity for keeping a stable tank. A good one is a bubble magus doser. They tend to be extremely expensive but in the long run, they are cheaper than all the liquid and solid supplements you have to put in your tank. They are a fantastic thing you can rely on to keep your tank stable. Keeping an unstable tank can lead to many casualties and deaths of coral and fish because your water parameters could be good one day and horrible the next. That is why you should save up and buy a calcium reactor or doser.