I notice that my yeast mixtures turns pink after a few days. I have not changed my mixture or anything and I think I'm not getting the same bubbling rate in the past. I used 1 cup of sugar, 1/2 teaspoon of yeast, and tap water in a 2 liter pop bottle injected into a cannister filter. This usually last 2 to 3 weeks. What could possibly be cause this pink yeast mixture? Thanks. Jonathan