The tap water in Europe is certainly much different than what we have in the US. A german hobbyist told me her tap water averages 25-30 ppm NO3 in the summer and the TDS is generally well over 600 as well.
However, as far as DPTA is concerned, it's designed for maximum uptake when the growing medium's pH is 6.0 or less. This may explain why many hobbyists report outbreaks of algae when they dose more DPTA chelated Fe.
TPN contains both HEEDTA and DPTA. HEEDTA makes iron available in alkaline water and therefore could explain why so many hobbyists have reported improvements when using TPN.
If I remember correctly, TPN+ also contains a small amount of ammonium nitrate, which is more immediately available to plants than potassium nitrate.
It's good up to about pH 7.5(assuming no CO2 is added), which is about where most folks are at. You might be confusing HEETDA with EDDHA, HEEDTA has a range of about 3-8pH. EDDHA goes up to around 11.
Still, both DTPA and HEEDTA are good for non CO2 and last longer in most aquariums than EDTA, which is the cheaper chelated iron. For us, the cost factor is not that large. They are only a few $ more or less and the amount we might buy will last a few decades. Gluconate is suitable for very soft water. EDDHA is good, but just a little bit will discolor your water, DTPA and EDTA, as well as HEEDTA are not too bad.
The bond strength goes from very weak, like with amino acids chelators to very strong like with EDDHA.
So DTPA might last 2-3 days in solution, whereas ETDA might last a few hours and Fe gluconate might last a minutes in say a KH of 5.
Dose = concentration X exposure time. So while the bond is harder to remove, the Fe is more available over time.
A mix of several chelators is generally better than one, but few notice much differences, they are in fact subtle, they are called trace elements for reason
If you want dosed NH4, feed your fish more.
Claus(past VP of Tropica) mentioned many folks in Europe also do not use CO2 or have fully planted tanks, but still dose for their tanks, things have changed since Tropica master grow came out some 20 years ago. But you simply add more to account for this, Claus was very adamant about folks increasing dosing if they had CO2, more plant biomass etc.