Phosphate Test Result Not Making Sense

[less than bread]

So, currently I only test Alk, Phos, and Nitrate. My Phos and Nitrate have been pretty high due to a period of negligence I went through earlier this year. When I actually started testing regularly and got back on top of maintenance, my Phos was 0.56 and Nitrate 55.7. Yikes. Since then through weekly water changes, those numbers have slowly been going down. Yay. 

 

Last week, I decided to also take care of a flatworm problem with flatworm exit. In the course of using this product, I replaced a lot of water to avoid the toxins killing everything. I probably put in over 30% new water.

 

Today I did my tests I do every Monday. Alk went up as expected and Nitrates went down a lot as expected. But Phosphate remained the exact same as before I added 30% new water. I plan on retesting later today, it just seems weird my phosphate has been going down steadily with normal water changes and then it remained the exact same after a larger water change. I use RODI water with Red Sea Coral Pro.

 

Also, my tank is currently fishless so I don't know where a source of Phosphate would come from over the course of the last week.

[seabass]

Phosphate can come from water (like from a RO unit in need of maintenance), it can come from food (powdery coral food as an example), it can come from dying algae (or really any organic matter), or the results might be due in part to test anomalies.  Could be a combination of these.  IDK, flatworm eXit caused some deaths, that could have some contribution.