Use Fe-EDDHA with UV-C.
UV-C causes oxides (like hydroxyl radicals) to form in the nutrient soltuion. The oxides destabilize and degrade Fe chelators, releasing Fe(III) (ferric iron) into the solution. Fe(III) tends to readily react with other compounds in the solution and then precipitate out of the solution.
I use a single pass UV-C, so I see a minor loss of Fe chelation with 50/50 Fe-DTPA/Fe-EDDHA. I would switch to 100% Fe-EDDHA, but Fe-DTPA has some advantages. I’m hedging my bets using a mix of EDDHA and DTPA, even though I doubt I would see a significant loss of chelation if I used 100% Fe-DTPA with a single-pass UV-C. If I used RDCW or other recirc systems, I would go with 25/75 DTPA/EDDHA, or 100% EDDHA.
Recent research found UV-C didn’t affect tested macronutrients, including NO3, K, Ca, Mg, and SO4:
https://www.tandfonline.com/doi/abs/10.1080/14620316.1997.11515498
@danielfp wrote two good blog post on this topic:
https://scienceinhydroponics.com/2010/08/iron-sources-in-hydroponics-which-one-is-the-best.html
When using Fe-EDDHA, look for the product with the highest o-o %. I recommend Sprint 138 because it has the highest o,o at 5.2%. Much more on the topic of Fe-EDDHA in my thread here: