High nitrates, no explanation. Need answers.?

I have two saltwater tanks, I have had problems with them before, but never this bad. They are usually between 5 and 10ppm, but today I tested and they were at 20ppm to start, I retested thinking I made a mistake then it read 40ppm. I did a water change today as well, I'm trying to get the nitrates below 5ppm for corals. But they rebounded and now it is out of control. I am not sure what I did, I have been doing this for years, I am not a beginner. Also Ca has dropped to 360ppm. It usually stays at a steady 450ppm. This is bad for my snails seeing how they need the calcium for their shells. Any advice or ideas would be appreciated. Is it my test kits? are they off? have they gotten too old? Also I have a 10 gall that has been cycling for a year now with live rock and sand. I tested the nitrates in there and it read 160ppm!!!!!!!! How is this possible when there has been nothing in there to create nitrogen gas for over four months? Three days ago when I tested it nitrates read 0. Do you think it could be my testing kit? I use API Saltwater, and it has always been accurate before. So any thoughts? BTW all other levels checked out as they usually do.