cal. mag. lvls droping

I am having trouble keeping my calcium and magnesium levels up to par. I do a 10% water change every two weeks but still have declining levels. I do not have any dosing set up but I have mix in supplements for these any thing I can do to help with the levles. Also on a side note my s.g. is falling too. I have a ATO I DONt KNOW IF THIS IS CA US ING THE TO FALL please let me know if you need any other info on the tank. Thanks