Now that I have your attention :-)
Full disclosure - I have an ATS, I have posted my build in this forum, and I have contributed to the knowledge base here. This thread is not about bashing any method. It is about trying to understand what is really happening, so that we can better our aquaria.
Abbreviations: P=total phosphorus, Po=organic phosphorus, Pi=inorganic phosphorus
Over at Nano-Reef we had a great, informative, entertaining, sometimes confrontational, but overall a very thought provoking thread on the concept that Macroalgae in a reef tank (in a fuge, or ATS) can NOT export net phosphorus. The origin of the Nano-Reef thread is one titled "Algae - What's Under The Hood..." over at The Reef Tank forum (you have to join to view). I won't re-hash everything in those 2 threads, and I encourage others to go through those on your own - they are tedious at times, but enlightening.
At first it seems impossible. How does an ATS not export P? What about all that algae I harvest every week that is thrown out, it has P in it, right? Yes it does...but during that week of growth, the algae has been dumping as much, or maybe even more, equivalent mass of Po (which we cannot measure) into the tank as it has been consuming Pi, feeding the bacteria that use the Po to make more Pi, which then feeds the algae, and so on...it is a cycle. The way I just described it is obviously very simplified, but for our purposes it will be suitable. There is research documenting this. The best one I have seen quantifies the net P biomass lost as 8% of the total algae P biomass - but this 8% comes during an incubation test period of about 4-5 hrs if I recall, and the algae were in a state of starvation so they were conserving Po. The references are in the above threads, I can post them later.
Algae will reach a homeostatic balance of Pi input and Pi export based on how much Pi is in the tank. It is like an algal survival mechanism. If it sucks up all the Pi available, it would starve (=not good). The algae won't keep extracting Pi at a steady rate until there is none left. It will balance its use depending on availability. It is mathematically impossible for an ATS to reduce total P in a tank.
There are other benefits to an ATS. But for this discussion, I want to only focus on the P export aspect of it.
What can the ATS do then, with regards to P? It can create a competitive situation whereby Pi is utilized by the ATS in favor of the nuisance algae in the display, which esthetically at least is a benefit. It can also act as a buffer, if you will. If you are a heavy feeder, the algae will increase its rate of Pi use, again preventing assimilation into nuisance algae, to a limited degree of course.
This leads us back now to the issue of the Po that the algae is leaking into the tank. On the one hand, Po does not really harm the tank inhabitants. It can even be viewed as a food source for coral. But it is also a preferred food source for bacteria. It is less energetically expensive for a bacteria to break P bonds in dissolved Po, than it is for them to break the bonds of Po that is bound to the rock or substrate (these are calcium-P bonds). Bacteria will utilize the high levels of dissolved Po in favor of the calcium-bound Po. What does this mean? The substrate and live rock can now more easily bind excess Po floating around. Depending on how big your P sink is (how much rock, how much sand), eventually, without outside help, it will be saturated ---> tank crash?
Water changes help removing Po. Siphoning and detritus removal take Po with it. A skimmer can be good at removing Po. Somewhere, in between all these methods of Po export, there is a balance that can be reached with an ATS in the system.
But the danger is if we rely to heavily on the ATS, then we are reaching a saturation point more quickly.
Now let's assume we are doing water changes, skimming, etc. along with an ATS in an adequate enough manner where Po becomes an overall algae limiter in the system (via reduced bacterial conversion of Po->Pi)...then theoretically, all things being equal, the ATS has reached maximum efficiency. This is good. It means that we have reached a point where the ATS will work the best in your favor to compete with nuisance algae.
I have seen numerous instances on this forum where algae growth seems to be reduced, or changes quality and color, and we keep pushing more light and more flow. Maybe this is incorrect - maybe in some of those cases, they reached the system max because they are exporting Po correctly? Maybe we need to reduce ATS growth because the screen/ light/ flow are exceeding the algal capacity? I would love to say, let's feed more, but then we are just making it harder to deal with all the Po.
I don't have nearly all the answers. I do have lots of questions, as I'm sure others here will too. Given this info about how P cycles in a tank, let's discuss and strategize.