My alk has been steady 8.9 - 9.1 on Hanna checker for past few weeks. I check almost every morning because I setup a two part dosing pump and figured I will check at least 4 or 5 times a week for the first month. I brought it up to this level from the low 8's and keep it there. Yesterday I vacuumed my sand bed and did a good sized water change prob about 15%. I do like 10%-15% weekly water changes. I tested my alk this morning and it was 8.0 dkh. The salt I used in the wc was the same mix that I have been using for at least the last month or so. I actually used the end and had to open a new box to mix for next change. I tested the dosing pump and it pumped an accurate amount. I opened a new bottle of reagent for the hanna checker and tested with two different reagents. The only change from regular weekly maintenance was that I vacuumed the sand bed in my display which I do like once a month. What can cause a 1 dkh drop in one day like that? Maybe something with the salt mix used for the water change? Any ideas? Thanks!