I do water changes every 2 weeks on my fluval 13.5 and I haven't been testing on a regular basis as I should be. Anyways I tested my alkalinity and it was at 6.7DKH but I tested my calcium and it was at 420ppm which is normal. What is causing this to happen and what should I do to fix this. (As I side note I should say that I have never dosed in the 8 months I have had this tank, just water changes.