Well, I just don't get all this NO3/PO4 paranoia, primarily because I haven't experienced the nitrate toxiciy syndrome and I've deliberately tried to induce it by hyperdosing. As far as I can gather the toxic effects of nitrate are as follows:
Poor growth, loss of appetite, lethargy, chronic stress, reluctance to breed, increase in gill rate and gasping, general ill health, delayed wound healing, clamped fins. I don't see any of this. My fish breed, have great appetites, don't suffer any more ill health that typical in this hobby and are unstressed as best I can judge by their coloration and general behavior. I lose fish to other reasons such as CO2 overdose jumping, aggression/predation and so forth. I don't keep inverts so I can't address those.
Furthermore I can't see how you can ever get to 120ppm NO3 if the water supply is held at EU regulated levels and assuming you do at least a 50% water change weekly. I've lived on several continents and the nitrate levels in the UK overall aren't any higher than any developed nation that has an intensive farming infrastructure, and despite the NO3/PO4 levels the quality of the water is higher than most places.
1) Lets assume your tap has 40ppm NO3.
2) Assume a 50% water change.
3) Assume a 20ppm NO3 via KNO3 addition weekly.
4) Assume zero NO3 uptake.
5) Assume zero NO3 production via nitrification.
Items 4 and 5 are necessary assumptions purely to illustrate the buildup rate as a result of dosing and water change.
So the tanks is filled with 100% tap water at 40ppm and is dosed with 20ppm NO3 so at the end of the week the the total nitrate level rises to 40pp+20ppm=60ppm at the end of week 1.
A 50% water change is performed which means that half of the 60ppm water is removed and is replaced with 40ppm water. The resultant concentration level is the average of the two: [60ppm+40ppm]/2=50ppm.
For week 2 another 20ppm is dosed to this 50ppm water so by the end of the week the NO3 level rises to 70ppm. After the water change the concentration level is [70ppm+40ppm]/2=55ppm.
For week 3 another 20ppm is dosed to this 55ppm water so by the end of the week the NO3 level rises to 75ppm. After the water change the concentration level is [75ppm+40ppm]/2=58ppm.
For week 4 another 20ppm is dosed to this 58ppm water so by the end of the week the NO3 level rises to 78ppm. After the water change the concentration level is [78ppm+40ppm]/2=59ppm.
For week 5 another 20ppm is dosed to this 59ppm water so by the end of the week the NO3 level rises to 79ppm. After the water change the concentration level is [79ppm+40ppm]/2=60ppm.
For week 6 another 20ppm is dosed to this 60ppm water so by the end of the week the NO3 level rises to 80ppm. After the water change the concentration level is [80ppm+40ppm]/2=60ppm.
Can you see that if you follow this procedure for the rest of your life you will never exceed 60ppm by the end of the water change? The peak concentration value will never exceed 80ppm. If you do more than a 50% water change then these values are reduced even further. Remember that this assumes no nitrate uptake and no nitrate production so this is purely theoretical. If the production rate exceeds the consumption rate then yes you will approach toxic levels. But if the consumption rate is higher then there is little to fear.
Now, if you are certain your tap water is 40ppm NO3 then really there is no need to dose KNO3, or at least you don't need to dose the baseline values. If you are certain your tap is high in PO4 then don't dose KH2PO4. All you really need to do is to dose K (most cheaply done via K2SO4) and traces. You are still doing EI as long as the nutrient levels in the tank are maintained higher than the uptake rate. If you start to see evidence of malnutrition then that means your assumptions were off and you can then make some adjustments. It's really no big deal. Fretting over toxic nutrient levels in my opinion is a complete waste of time and it really sucks the energy and enjoyment out of the hobby. There are a lot more toxic things in your tank to worry about like urine, feces, other organic waste and pathogens. Keep those concentration levels low and you'll be ahead of the game.
Cheers,