It takes a balance of Alkalinity, Calcium, and Magnesium to keep this from happening. I hope you're testing your water to see what these levels are. Never add anything to your tank before testing to see if it even needs it, and always test afterwards to make sure you aren't under-dosing or over-dosing. Make sure your Magnesium level is between 1250 - 1350 ppm, as this is the buffer that keeps Alkalinity and Calcium ions from bonding in the water column. If they bond in the water column, you end up with what you have now. Also... your Alkalinity level should be between 7 - 11 dKH, and Calcium level between 425 - 450 ppm.
If you don't have test kits for these three elements, you should get them. The health of you tank is dependent on the quality of the water, so don't skimp on these kits. Cheap test kits result in faulty readings. Get the good kits. Before you add anything else to the tank, you should create a chart and test every few days. Write the results of your tests in the chart. This is important because it will show a pattern of how much of each element is being used in certain time frames. This is how you determine how much of each element to dose. Guessing doesn't work in this hobby...