What’s w this mix?

Hello All!
I was using Sterlite bins for mixing WC’s for years to do my mixing. Never once did I give it a thought. #5 plastic so, it’s food safe at least. Also,I had been using standard Home Depot buckets. They have a #2HDPE(High Density Polyethylene). From what I can see that is better than 5.
I had put my TDS meter in the Sterlite mixing bin. With fresh water my TDS was reading .019ppm. Not really all that alarming. WHEN I put the meter in my Home Depot bucket for my ATO IT WAS 000.
I wasn’t sure if my Cyano in the DT was coming from the mix bin so I tested the PO4 after it was mixed with my Hanna checker. I made a batch of fresh SW,tested the PO4 and got .03. Which is fine I suppose. It’s way better than the .17ppm I was at. BTW I am using Fritz’s Blue Box.
I went to tractor supply and replaced both my ATO bin which wasn’t really necessary and my mixing bin. Bye bye Sterlite. Both new bins are HDPE#2 plastic. I put FW in the mix bin and checked it in an hour. .006ppm TDS. Then, I made a batch of SW and tested the PO4 for giggles. .03ppm. WTF?
SO, is it the salt? Would the TDS contain some for of PO4? Is my checker bad? It’s brand new. I realize it’s a long post but Thoughts?
Some salts contain Po4. They are made for those who keep tanks over filtered and relying on constant water changes. Not sure if fritz is one of them. I would email and ask.