why not test RO/DI water? Should be zero right? So if the Hanna tests over it should be pretty obvious.

Hmm good idea. Worth a try.

I don`t think po4 shows up on tds meters so even if you have 0 tds you could have po4
I had a hanna phosphate checker and it was a POS, I thru it in the garbage, I have ordered a hanna phosphorus ulr as i read they are much more accurate, they measure ppb phosphorus which is mathematically converted to po4

Ive checked both my tap water and ro/di , tap was 0.11 , ro/di was 0.00 .That's why we use ro/di it takes the phosphate out of the water , phosphorus is a solid so it would show up on tds . I was surprised first time I used my hanna meter but with proper procedure open the little packet first and empty it onto some wax paper, cleaning vials with vinegar and ro/di , I got very consistant readings.

Received my phosphorous checker last night. Will update when I have more testing done.