Yup i think its low nutrients too,
Alk of 5 is very bad but corals are affected more by low nutrients than low alk ..
Also check magnesium, unless magnesium is correct levels you cant get proper levels of alk and calc.
Start feeding more, that too do it slowly and then use some method and raise alk like 0.3 dkh or so, anymore and you would see stn from tips
Curious, did you mix your salt bucket when you opened it ? Chances are its been sitting for too long in stores and all magnesium settled to the bottom and can cause problems like this .. If you haven't mixed it, please do so now and test for alk, calc and magnesium in fresh batch of salt water
Sent from my iPhone