Unless your tap water is really high in phosphates there is no reason to do anything except say thank you when you find phosphates in the tap water. That just saves you the trouble of dosing it for one dosing period. Really high means something like 7 ppm or more, maybe even 10 ppm. And, you need to very carefully calibrate your test kit if you are going to use it to determine how much phosphate you have. That takes some effort.