I nuked my tank tonight, several hours on and things look like they are still 'fizzing.' Is this normal?
It's normal. When H2O2 reacts with organic material, it produces oxygen - lots of it. The H2O2 is essentially gone at this point, but it takes a while for all the excess dissolved oxygen generated earlier
to slowly bubble out of solution.
The 50% water change, if performed with tap water, is also loaded with dissolved gasses. Chances are, you've heard of or witnessed "false pearling" after a water change.
In this case, because both
the old and new water are saturated with gasses, it can be quite a show.