The 50% weekly water change prevents accumulation by creating a "steady state" whereby the maximum concentration of nitrates can't go above a certain point. Here is a graph generated from
this calculator.
This is a graph of nitrate concentration using recommended EI dosing. If there is 0% nitrate uptake, then concentrations will reach a maximum of ~45 ppm. If you tap has ~45 ppm nitrate (a gross violation of EPA standards), then it will reach a maximum of ~75 ppm, assuming absolutely no plant uptake. At 50% uptake, you see a peak of a very reasonable 25 ppm, closer to ~50 ppm with 40 ppm tap, which is pretty reasonable considering your tap.
That being said, the premise of PPS is screwed a whole lot harder by excessive nutrient in the tap than EI is. If you have 40 ppm of tap nitrate, you could just cut nitrate from your regimen and call it good. PPS is shot at that point. Really, in that light, EI is better suited for situations where you have high tap nutrients as it's much more flexible.
Second, regarding your point about 'guessing' rather than relying on test kits.
First, something that a lot of people don't realize is that the accuracy/precision of our testing kits is really quite bad. There are technique errors inherent to liquid test kits (you
need a calibration curve to have any degree of confidence in your numbers) and user error is probably more rampant than anyone appreciates. Therefore, any technique that relies extensively on test kits is difficult to properly apply. In that regard, techniques similar to PPS are much more limited than EI will ever be.
Second, the thinking behind EI challenges the entire premise of PPS. If PPS shoots to provide 100% nutrients per day, no more no less, than what happens if you have 120%? Not much, as we see in essentially every single non-PPS tank. If you have
any nitrate or phosphate left day to day, then plants are getting 100% of their daily intake. However, algae have a much lower threshold of nutrients. Whereas a plant may need 1 ppm worth of nitrate per day, algae may need .01 ppm, allowing it to much more easily get its daily intake. Thus, it doesn't matter if you have 10 ppm or 100 ppm, everyone can get what they need. In that regard, PPS and EI achieve the same thing in the end: plants have 100% of nutrients at all time. The main difference is the 'steady state' mentioned above, where PPS strives to maintain much lower levels at the cost of simplicity, and EI takes the honey badger approach (ie, it doesn't care), eschewing the need for low nutrient at the cost of being
extremely user friendly.
Essentially, it comes down to this question: is the complication/technicality of PPS worth the low nutrient accumulation? Inherent to this is another question: are high (really, only moderately high, mid 30s NO3, <10 phos) going to harm a tank? There are clearly situations where the answer is yes (eg, discus, SA breeding programs), but for the majority of situations, the answer is no (in my experience/opinion).
Oh dear, I appear to have babbled on a bit.
