hallo i would suggest you drain the water and replace with a mixture of anti freez and water, the mixture should be a greenish colour, if you still have a problem you might want to think about seeing your mechanic.
caltex extended life coolant is great. Mix to instructions with distilled water.
Just be sure all the previous coolant has been flushed properly as some coolants create a gel when mixed together that blocks radiators, small passages and heater matrixes.