Can Kh go from 20 to 2 in 5 days?

[Rogue951]

First off I admit I'm a careless doser and tester. PurpleMonkey will attest that my tank seems to be a freak and get away with things most other tanks don't.
Most consistant I dose Brightwells 2 part.

I randomly tested one day and found my Kh to be 20. So I stopped all dosing (Except Kents Mg... trying the nuke the algae theory)
4-5 days later I test again and my Kh is supposedly at 2. I tested 3 times with a Seachem kit.

there are NO noticeable differences to the tank inhabitants. I've noticed no unusual precip of powder...
can I really use up that much alk in 5 days?

40 gallon breeder w/sump - approx 60 gallons water.
heavy on the SPS, LPS and 2 clams.

All other parameters normal.
Ph 8.3
Nitrate, ammonia, nitrite = 0
Phos - 0.05
Mg 1900
ca 500 (also dropped from 700 5 days ago)