Calcium Reading Over 500

[DigitalWeight]

Tested my water yesterday and today and am seeing reading of over 500. Thought at first it could be a bad test kit, but ran the test with both a Red Sea kit and a Salifert kit and got the high readings.

Here are my parameters:

Salinity: 1.026
Alk: 7.4
pH: 8
Mag: 1500

The salt I am using is aquavitro salinity.

One thing I have been doing is doing a 1 gallon water change every couple of days. Mixed reef tank with not a lot of big colonies in there.

Any ideas on why it would be so high - I have some white particles floating around so I think it might be parcipitating out but not sure. You can see the tank here: DigitalWeight