Get ahold of your water company water quality report, then look to see what range of phosphate amounts they say is in the water. Or, phone that water company and ask to speak with someone in the testing department, and ask them what the range of values for phosphate are. That range will be far more likely to be correct than your test kit will.

Or, calibrate your test kit, by mixing up distilled or DI water with KH2PO4 or a similar phosphate compound to get a calculated concentration of 100 ppm of phosphate, for example. Then take 10 ml of that water and add 90 ml of distilled water, which will give you 10 ppm of phosphate. Check what the test kit tells you is in the water. Then take 50 ml of the 10 ppm water and add 50 ml of distilled water to that, giving you a 5 ppm phosphate sample. Check what the test kit says is in that water. Repeat by mixing 20 ml of the 5 ppm water with 80 ml of distilled water to get a 1 ppm phosphate sample. Those tests will tell you what your test kit really means by its readings. Assuming the kit is found to be accurate, only then can you trust your test kit.

Even if you actually do have 5-6 ppm of phosphate in the tap water you can't be sure that is enough to last a week with the plants using it up. You could skip the phosphate dosing right after a water change, but go back to dosing for the next fert dosage scheduled. In any case, 10 ppm of phosphate isn't going to cause an algae bloom. A spike of ammonia or an unstable CO2 concentration from day to day is likely to be the trigger for the green water.

Thanks Brian!
Good supplier!

Regards,
Tom Barr

Got my answer from the "high phosphates" thread about the phosphates.

Any ideas about the K?