What type of aquarium are you running? i.e. south american, african, etc. My Quantitative Chemical Analysis textbook shows that EDTA is capable of chelating metal ions at various pHs, depending on the major species of EDTA present (metal-EDTA complexes become less stable at lower pH ranges.)
Also, you can use any pH value to select which metal ions you want to be chelated by the EDTA and which ones will not be chelated. (example from my book: if a solution containing both Fe3+ and Ca2+ is titrated at pH 4, the Fe3+ is titrated (chelated by the EDTA) without any interference from Ca2+)
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