Yeah, that 3500 was our starting amount... we ended the year at 4000 last summer, so I thought that the -500 represented winter loss? Wonder if that 3500 was just a low initial reading and really we opened closer to 4000?If you had salt in the water left over from last year, it does not go away so that might be it.
Is your SWCG running? That's a possible source of salt. Are you using test strips instead of the test kit? That's a problem too.
No test strips here. . . generally just go with the sensor readings, though I have a salt test kit on the way. . .