Actually, just after I wrote that, I recieved the answer to my question. Over night my water went from OK to bad. Up to that night, the water was very slowly taking more chemicals to balance and the clarity was slowly getting worse...not cloudy, but not sharp, and no smell, but I was still able to manage the water.
My wife and I had another couple over for about a 2 hour soak (aka, the three martini soak) and that made the water flip flop from OK to bad. THat night, I excessively shocked the water and went to bed, the next day I checked the water and it was done. Cloudy, smelly, foamy...stick a fork in it, it's done.
I guess my question now is, does it normally go bad that quick? Or should 4 adults (I use the word loosely) not spend that much time in a 400gal tub?