Hi Viktor,
But then based on your 10ppm feedback above i may will do some additional tests to confirm it.
Almost everyone thinks that they have CO2 beaten. I never assume this. I always assume that there is a CO2 issue just around the corner,
Adding nutrients drives a DEMAND for additional CO2 uptake, and adding CO2 drives a DEMAND for additional nutrient uptake.
So, while the CO2 may have been OK before you added the extra PO4, once you increase the consumption of PO4 the plant then needs more of everything, including Nitrogen. Increased Nitrogen uptake is a much easier problem to solve and you may not have even noticed the Nitrogen uptake increase because your sediment may have already been unlimited, however this would have then increased the CO2 uptake demand. If CO2 was not unlimited you would have crashed the barrier.
I routinely dose massive amounts of PO4 to the water column without any issue. Here is a very old thread showing the results of super massive PO4 dosing=>
http://www.ukaps.org/forum/threads/blyxa-japonica-flowers-underwater.1967/
So no measure since there aren't anything we can use to measure just throw in and watch?
Well I suppose you could try to calibrate the kit, but honestly, PO4 test kits can only ever detect the presence of PO4, they can't tell you what the measured value is. It's exactly the same with NO3 test kits. They are easily fooled by other ions in the water column.
The thing is that apart from knowing what the tap water content is, which we can't really tell, if you are dosing a certain amount of the powder then it's very simple to calculate what you have added. So although you cannot account for the content added by the municipal supplier, or the amount added by food, you can easily calculate, with great accuracy, how much you have added if you are adding a salt like KH2PO4.
People get into trouble all the time assuming that the PO4 test kit is telling the truth, and in almost every case, the test kit over estimates the PO4 content, and they are so afraid of PO4 boogie man that they avoid adding PO4 which then causes more problems.
Cheers,