It depends on the media being used. I’ve only ever used Carib sea arm. Which takes care of cal,alk,mag plus trace elements.
ive never used reborn, but have read about people mixing magnesium with it (if you can find it)
also It really depends on your systems uptake, some use a combination of dosing, reactor, kalk to maintain their systems requirement. However I think both you and I are a long way away from that.
short to mid term answer, yes a calcium reactor means you won’t have to dose.
I also find with my reactor my dkh and calcium stays pretty steady and stable.
also if your just starting out with frags, water changes may be enough. I only employed a calcium reactor because I had one from years and years ago and kalk wasn’t enough to maintain my requirements. plus I hate dosing.