Hanna alk testing high

[Chadrph]

My alk has consistently ran at 10.5 using my Hanna checker. However for the last week my checker has been giving a consistent 11.7 reading and has not changed. I stopped dosing 2 part for the last week assuming my son had overdosed the tank while I was on vacation, but the reading has not come down. Tonight I did a double check with my salifert test and I got a 9.5 reading on 2 tests. Does anyone know what could be causing the high readings on the Hanna checker that are not changing despite not dosing my tank?

Thanks in advance.

 

[Reef.]

Mine did the same, started of reading the same as my Salifert, now it is consistently 1dkh higher.