Another way that I've come to think of tank dynamics is by analogy of human needs and our environment. For example, a plant under strong lighting is like an athlete. When athletes are exerting themselves, they need more O2. They achieve this by breathing faster, which pumps blood faster, which transports O2 faster. Similarly, under strong lighting, photosynthesis is pushed to the higher extreme and the plants require more CO2. Depriving the system of enough CO2 would be like the amount of atmospheric O2 being depleted from the air we breathe. It's like what happens at higher altitudes and people who are not used to it getting light-headed.
With regards to nutrients, athletes need to assimilate enough food to replenish their energy stock. They use it up faster and need to replenish faster. Same with plants. If you run a tank low on macros, it's like an athlete not getting enough food to maintain peak performance. If food is deprived long enough but the athlete is forced to perform all the same, s/he will starve to death quickly. A sedentary person doesn't have these requirements; Take me, for instance

: I can subsist on minimal caloric intake and I could probably be shot up to the top of Mt. Everest and not feel too woozy compared to a hungry athlete who's doing jumping jacks on the way up.
And last but not least: traces. We all need them as much as we need food. But a high-metabolism athlete needs more iron (among other minerals) and vitamins for the proper development of muscle tissue and again for O2 transport (via hemoglobins) than us slow-moving average folk who probably store iron in excess by the consumption of way too much red meat. With more than 2 wpg of fluorescent lighting (just as a rule of thumb), it's good to supplement (both CO2 and nutrients). PC lighting is more intense and at 2.5 wpg and above, it's like feeding a team of Olympic hopefuls. Providing too much or too little of anything is going to lead to an undesirable outcome.
Many claim that 2 wpg of "normal" fluorescent lighting is enough to grow almost *any* plant. I do think that some plants look best when "worked out," however. In any case, it doesn't hurt to have a little extra CO2 available via yeast generator or Excel, and a little extra nourishment to keep some meat on the bones. And I'm still rambling.
Happy planting to all noobs, and welcome to this wonderful obsession!
-Naomi