[Delphinus]

I like to think of it this way.. Testable levels of anything are sort of like the level in a tank that is being emptied and filled at the same time. If the tank is being drained faster than it is filled, the level will be going down, or possibly even zero if the trend continues long enough. If it fills faster than being drained, the level will be rising, unless it rises to a point where the higher level pushes water out the drain faster and thus reaches a steady state.

A test reading is a snapshot in time of where the level is "at that moment" but to determine if there is a trend you have to take many snapshots over time and compare.

So taking what we know into account (like "fish poop from time to time") we know that ammonia is produced but we never get a reading in a healthy tank because it is consumed as quickly as it is produced. The bacterial strains that consume nitrate however, take longer to establish and are less hardy than those strains which consume ammonia and nitrite. Those that consume phosphate even less hardy.

I guess what I'm getting at after all this is that just because a reading is 0 doesn't mean that it's not the explanation for something. You could have a nitrate or phosphate reading of zero and yet algae is in the tank or coral growth is affected.

Not saying that testing is not worth it (far from it in fact) but it can only show you so much. At some point you have to rely on your instincts and intuition as well.