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Answers       share|improve this answer add comment  the only
possible reason for the water loss, and that if you're not
getting any noticeable leakage, is Improper Venting, as
described here. there is another reason why it sound that
Improper venting is the cause - you noticed that the water level
was getting lower over a year period. it could be that over that
year your plumbing venting clogged slowly & steadily.  Also,
contrary to what said in the link above (that in a case of
improper venting you need to call a plumber), i think that if
you've noticed the change over a long period, this is also
reversible (bird nesting or something similar). if the problem
was very low but steady water level, then it could be a problem
that require a plumbers work.  share|improve this answer add
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