Tap water with 5 ppm of phosphate isn't going to be enough to satisfy the plants, unless you do water changes often enough to replenish the phosphates the plants use. A single dose of any of the ferts isn't going to be adequate. If I thought I had high phosphates in my tap water I would skip dosing phosphates for the first after water change dosing only. After that missed dose I would just dose normally. Phosphates in the tap water are just free fertilizer, not something to worry about.