Welcome to TR.com first off
Im not sure how pure it is but when i was setting up my tank i used at least 50% of water out a foset brita. and when i went to messure the levels i could messure cal or
alk. calcium was showing up as not present and so was
alk. so i drained the tank and tried it again. the same way. well i got the same results. the third time filling the tank i used
RO water and everything was fine. im not 100% sure that was the reason but its kinda funny how it worked out. so i havnt used the brita for my tank since..