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 Seachem salt will often cause inaccurate kh readings because of the high levels of borate.  Most test kits read the combined total of borate hardness and carbonate hardness giving a higher reading than what the actually carbonate hardness is.   When I used the seachem  test kit and subtracted the borate I found that my actual carbonate hardness was about 2 dkh lower.  So your kh is probably a lot lower then your already low reading. 
				__________________72 gal bowfromt mixed reef sps dominated, 25 gal mineral mud type sump/refugium  Skimmerless
 2x250 14000k phoenix hqi  2x96 pc actinic, 50x flow
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