If you're within the pH parameters for EDTA to remain stable it works extremely well.
I had great success with it full EI as per recommended instructions when I was running a tank with near 0dKH and had an active substrate (Fluval Stratum Shrimp) where the water pH profile was from below 6.5 down to 5.2ish (or lower). The problems started for me with EDTA when somehow the tank water KH rose 12 points in the space of a week, wow that was a wacky week, had no idea what was going on until the plants started to look really bad and then I tested everything and saw the KH pegged out when it should read near zero, on reflection I suspected the stratum substrate just disgorged all or any KH it stored up over time after a micro dosing accident (200ml got dosed because I got distracted and forgot I had the autodoser running, did water changes to rectify it but the substrate must have been done for) and the pH rose taking it out of range for the best use of EDTA. Remineralisation here was entirely Bee Shrimp Mineral GH+.
It didn't stop me from using EDTA though, I continued to do so but fortified it with DTPA to take into consideration that after a total restart (for Bucephalandra) I was going to add KH to bring the water to a more neutral pH (Seachem Alkili alongside Bee Shrimp GH+), stem plants always did well but the Buce always struggled with deficiencies or melted away entirely either bottom up or top down. Reading as much scientific literature as I could find on Bucephalandra and a side note anecdote (not supported by any measurements) of 'Sensitivity to Metals in water' gave pause for thought specifically because the mix of salts in a bag of EDTA trace are non homogenous, the ratio of elements could be very slightly out of whack in each teaspoon especially more so when you aim to get some copper sulphate crystals in (because that's really the only element apart from the iron that you can differentiate from the other salts), thus trying to pin exact ratios down is fruitless unless you mix the entire bag in one go into water (providing the bag has had weighted elements in a known exact ratio added to this bag and not taken from an even larger bag where each scoop may have a slightly different ratio), in the grand scheme of things this shouldn't really be a problem unless you are trying to pin down a particularly sensitive element (Cu?). So I looked into it and went about doing things differently and because my concentration was about Buce I specifically looked at how they get their nutrients in the natural environment and what their sources are. (I should note that my Remineralisation strategy here changed and I replaced the Seachem Alkili for Potassium Bicarbonate, not because it wasn't working but because I ran out and was too stingy to pay for more).
From where I could find analysis about water where Buce are likely to live Copper is present in very trace amounts where it is noted as <0.00mg/L (it's not saying it's not present just likely that the test doesn't detect it below this level) but Manganese could be as high as 0.05mg/L, so in the first instance I fortified my EDTA/DTPA mix with extra manganese (because my plant symptoms appeared to indicate K deficiency (pinholes in leaves) alongside Ca deficiency (loss of structural integrity, fancy way of saying melt) when they were present in abundance, literature appeared to point he finger at manganese for these combination of symptoms. This actually helped a lot and temporarily alleviated some of the problems but never entirely, root health was always an issue so I started adding phytohormones to help out the roots (I'm growing Buce entirely epiphytically on Bogwood and the roots have no access to an interstitial soil) again this helped but things still weren't right. Around this point I changed things up and started front loading the macro weekly at water change and dosing micro daily.
Staring at Mulders chart (often) and trying to match it with real numbers (you'll get through a lot of fertilisation literature doing this and come across an enormous amount of data not always correlative) I came to the conclusion that antagonism really is a thing and its ratios not total concentrations that work and you also need to pick the appropriate micro chelate.
Determined to fix this situation I decided to stop using EDTA and swapped to Flourish Comprehensive fortified with DTPA and Flourish Trace, spiced up a little with some extra manganese, all dosed beyond their bottle label recommended levels by targeting EI micro daily rates. At this point for remineralisation I reduced the potassium bicarbonate levels by using powdered cuttlefish bone instead which reduced the need for Bee Shrimp GH+ (down to a pinch for the chlorides) and started front loading macro weekly with extra magnesium sulphate. Things were going in the right direction and I was seeing huge improvements in Buce health by ratioising everything by this point, but it was going to be seriously draining on the wallet to do so using Seachem Trace (using because it's non chelated for low persistence), so decided to DIY everything including my simulated Karst water recipe were discussing here.
I'm 4 weeks + into no water change with my daily micro dosing, (micro daily level listed way above, copper is in this mix, have dosed extra MgNO₃ and KH₂PO₄), GDA on the glass and BBA increasing but I'm not seeing any signs of toxicity, I may or may not have accumulation happening, however my equilibrium pH is now 7.95 up from 7.8 (no extra KH added, inert substrate), my other tank (getting same treatment, ert substrate) has risen by the same 0.15 points, so over 4 weeks the chemistry has slightly changed, the daily micro solution is acidic at pH3.2. I run electrocoagulation at night so I am ruminating that hydroxides may be accumulating (without testing, no idea).
