No one knows exactly why they bubble but my theory has always been that it relates to temperature. In my old apt, we kept it at 70 degrees with a tank temp around 76 and I had beautiful bubble tips. One day I decided my tank was getting to cold and turned it up to 78 and the bubbles stopped. Here in my parents house, the home is 78 and tank hits 79 on average and no bubble tips in 3 months.