Seachem iron is iron gluconate. Iron gluconate is not PH sensitive like the other ingredients. It will work at any PH. But it doesn't last long. Gluconate is a type of acidic sugar. So bacteria will consume the gluconate and then the iron turns into iron oxide which is not soluble in water and therefore unusable by plants. How long iron gluconate lasts in a aquarium is impossible to predict. The general recomendation for iron gluconate is to monitor your iron levels and add iron as necessary. your yellow leaf with green veins is common for iron deficiency.The big light-green leaf (main leaf) is an old leaf. The Iron that I dose is Seachem Iron which I believe is ferrous and my pH is typically between 8 and 8.2.
The other iron fertilizers used are PH sensitive:
Iron EDTA is stable at a ph of 6.5 or less. Above 6.5 is converts to iron oxide.
Iron DTPA is stable up to a PH of 7.5 but some is still available up to a PH of about 8. Again once the PH gets too high the iron converts to iron oxide.
Iron EDDHA is stable up to a ph of 10 or 11 which would seam to make it the ideal iron fertilizer. However it is the most expensive and it imparts a red color to the water. Due to the water color issue it is seldom used in aquariums.
For most aquariums a dose of 0.1ppm if iron is sufficient for a week. My PH is about 7 so I use DTPA. Iron is never a problem.
Sodium in an aquarium isn't actually bad for for plants. However in a farm field Sodium chloride will prevent plants for absorbing water. But in softened water the sodium is in the form of sodium bicarbonate not sodium chloride. Sodium bicarbonate will push the PH and KH up and it will react with any fertilizer in the water. Many fertilizers have manganese, zinc, and copper sulfate. Mix this with sodium bicarbonate and you get sodium sulfate and insolble manganese, zinc and copper carbonate which plants cannot use. it might also react with iron gluconate.I had a feeling you were. Use regular tap water. Softened water strip out calcium & magnesium and replace them with sodium Which is bad. Plants need Ca& Mg and large amounts of Na blocks plants from up taking nutrients.
If you are using fertilizers you want to keep your KH as low as possible.. And the easiest way you have to do that is to stopping softened water.