The article is a bit old. IO has boosted the calcium and magnesium in the mix, and at least magnesium isn't needed. At the moment I do not boost it with either.
Reef Crystals is a fine mix, however, but I still prefer IO.
I'm skeptical that the tap water really has 25 ppm phosphate. But even if it does, an RO/DI will remove it adequately.
When you ask about RO, do you mean an RO/DI? If you are getting 0 ppm TDS out of it, three's no need to test it for anything. A trace of phosphate in the RO/DI water isn't any concern. The spectrapure unit is likely a fine unit. I have one made by them as well.
i discuss it here:
Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both ? Advanced Aquarist | Aquarist Magazine and Blog
from it:
Comparison of Food Sources of Phosphate to Other Sources
What about other sources of phosphate, like the "crappy" RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.
Let's assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.
