Polywise
Well depends on your volume of water and your bioload.
Here is and example
240 reef tank,my reactor went crazy on me.Went you to 11 alk and of coarse Cal went up also.I just turn the reactor off.And just play the waiting game.You will be very surprise how long it takes to fall.But Alk and Cal will fall very slow.This is the best for your corals,the slower the better.It took my system 5 days to get it down to 9.6,from 11.So don't be concern how long it takes.Just keep testing with your test kits.You really only need to check Alk,because Cal will fall along with it.It depends where you want to run your system at.But just let it drop on it's own.You need not to add anything.I was very surprise how long it takes.You would think by turning it off,it will drop fast.But it doesn't.So just chill out,and let the tank do it's thing for you.If it's done to fast,big issues with STN /or RTN.But doing it this way,this is the safest way,to get back on track.