"Conclusion
Foods are by far the most important source of phosphate in most aquariums. While there are big variations between foods, it does not appear in this analysis that dry foods are the nasties they are often made out to be, relative to frozen foods. There are better and poorer choices (with respect to phosphate) to be made within each food type. Avoiding foods with bones, however, might be worthwhile if delivering less phosphate is a goal. Additionally, fresh grocery store shrimp seems to be one of the best foods from this standpoint.
In considering whether sources of phosphate other than foods are important, one must carefully look to the actual amounts involved to determine whether other sources are even worth trying to minimize. It can be scary to learn that your purified fresh water has phosphate in it, or that your salt mix has detectable phosphate, or that your supplements or whatever have some phosphate. But just because you detect something, and maybe you even detect a concentration far higher than in your aquarium, that does not by any means imply that those sources are significant enough to warrant some sort of corrective action. Our analytical tools have become fairly sensitive, allowing us to detect things which might sound like trouble, but really aren’t. We need to understand the various dilution issues involved as well as the overall phosphate balance in a reef aquarium to evaluate the importance of different measurements.
Just use some math and put it all into perspective, before using some dollars or time to chase a trivial “problem”.
Happy Reefing"
Ultra pure water out of RODI will fool many test kits as it doesn't have the ions needed to make the reaction happen that the test kit is looking for. The bold at the bottom is the important part. Phosphate comes from food at 100x the rate of other methods, so that is your culprit. Feed less, GFO, lanthanum chloride, ATS, a big fuge will all work and have their own quirks.