This is particularly a tea issue. And has only been occurring for the past few weeks. But I've noticed that certain cups of tea fizz. You can hear them fizzing and they develop a coffee-like froth around the rim of the tea surface. We've tried various brands of tea, from Tetleys to Clipper organic, plain tap water, filtered, spring water. Washing up liquids from Fairy to Ecover. The phenomenon appears to occur more in 'straight up' mugs rather than inverted bell shaped vessels. I haven't been able to pin this one down, but suspect excessive amounts of aluminium sulphate applied to the water. Any other suggestions?