The tank is full of bubbles because your presumably male betta fish is very happy with his environment. When a betta fish builds a bubble nest, it means the conditions are right for him to mate and he feels that his offspring will survive. There is nothing to worry about. This is a good thing!
After tap water is dechlorinated for fish use, it should be left to sit for a while so that the gas bubbles settle. If you don't do this, it will not hurt the betta, but bubbles will be apparent. They will go away in time.