Sorry, my bad. Bob sent me the following shortly after Thanksgiving and I totally spaced posting it.
Thanks for informing me about the thread on MR regarding our ESV Salt Mix and phosphate. At ESV we use the HACH DR890 Colorimeter for measuring orthophosphate. The method we use is Method 8048 and is USEPA accepted for wastewater analysis reporting for samples containing 0 to 2.5 mg/L phosphate. This is what many would consider a "high end" colorimeter and allows for calibration and accuracy checking with NIST orthophosphate standards from HACH. To get optimum accuracy and precision, this method requires meticulous technique. Sample vials should be spotless and acid washed. The proper cloth must be used to dry them. Paper towels for example, can leave micro scratches and effect light transmission and therefore results.
To speed up production, we cross reference a HACH PO-19 Color Disc Orthophosphate kit (runs about $100 from Hach) with the DR890 and use that for the bulk of our quality control measurements. I have also determined the Salifert Phosphate kit run with their "higher sensitivity" technique to be acceptably close to the HACH products and use that on my home aquariums. In fact, once below 0.05 ppm, I find the Salifert high sensitivity technique a bit easier than the PO-19 to hone in on very low levels. I haven't evaluated other kits so I can't speak for their accuracy. Given the technique required to get meaningful results with a colorimeter, and the inability for many of them to be calibrated, I generally do not take too seriously the phosphate readings I hear generated from the more "economical" colorimeter devices on the market. I take the readings from the HACH PO-19 and Salifert much more seriously.
Typically batches of our salt range from 0.02 to 0.04 ppm orthophosphate using our test methods. Although we take steps to lower phosphate levels of our liquids, we get a very small contribution from the USP Magnesium Sulfate we supply. I'd hate to have to change our source over a trivial amount of phosphate because of all the positive feedback we've been getting on how corals are responding to the product. Unfortunately earlier in the year (before May) we may have had a batch or two slip through our quality control and fall within the 0.10 to 0.15 ppm range. The source has been identified and corrected. If any aquarist on this thread thinks they have a batch measuring over 0.1 ppm orthophosphate, I would be interested in hearing from you. I'm usually available to take calls (800-846-9449) weekdays between 10:30am and 4:30pm. It would be helpful if before calling you verified any phosphate readings taken with the more economical colorimeters with the Hach PO-19 kit or Salifert and of course the results of your RO/DI water taken with the same kits. I'd also like to know the batch number of the Component B liquid.
If it turns out you did receive one of those higher phosphate batches, I don't think it's a serious problem in the grand scheme of things. The amount of phosphate added to your system via the salt mix would be trivial compared to phosphate entering via feeding. Randy Holmes Farley wrote a great article putting this into perspective:
http://www.advancedaquarist.com/2012/3/chemistry.
From the information in that article, one could conclude that doing a 20% water change on a 100 gal aquarium which tested zero for phosphate would only add 0.03 ppm orthophosphate to the system if the new mix tested as high as 0.15 ppm. That phosphate addition would be equivalent to just one days feeding for a lightly fed tank or only one tenth the daily phosphate addition of a more heavily fed system. A newly mixed batch of salt measuring 0.04 ppm would have essentially no significant contribution compared to food input, especially if the aquarium water typically ran at 0.04 ppm. We must also factor in that phosphate input from food is usually a daily occurrence and 20% water changes usually don't occur more than once per week. The big issues with phosphate control, IMO, are input from food vs export efficiency along with possibly identifying an internal "sink" (old undisturbed sandbeds or other areas where detritus can accumulate) which has built up over time and may be at a point of inputting orthophosphate back into the water column.
I hope this sheds some light on this thread and thanks to all of you who support ESV. If you have further questions please feel free to call me. I enjoy talking to fellow aquarists.
Bob Stark
ESV Aquarium Products Inc.