JamesW nailed it. I was replying from my phone earlier, so I didn't say all that I wanted, the tube turns blue over time so I'm sure that can skew the results and then it is subject to the person that is reading the test. I've had a customer with 1000+ ppb phosphates, somehwer around 40k gallons, took 5 qts, of hasa phosout, removes 1500ppb in 20k gallons, but that did get him below the 200ppb phosphate level, and it has stayed down to, saw him a week ago and tested his water. Just be diligent with the products they try to sell you to remove phosphates. Some by the natural chemistry are weak, 600ppb in 20k gallons, like i said the hasa 1500 ppb, seaklear has one at 3000ppb in 20k gallons, and they have a commercial line the will remove 9000ppb but it will also remove a good portion of your wallet.