Plant Deficiencies

Could you have a look at the horizontal peace Lilly leaf on the left of centre please. The photo is very burnt out but there are two yellowing leaves on the plant that are showing what I guess is a deficiency of some sort.
I add 4ml of TNC Lite each day into a 300litre tank, it is Nitrate and Phosphate free as the Fish maintained Nitrates between 20 and 30ppm.
The lights are on the same cycle as the tank, 8 hours per day with a siesta, I don't think it is related to light levels as there are fully green leaves both closer and further away from the lights.
I inject CO2 and it is currently at 19ppm, although I would also not expect that to affect emerged plants much.
I have been fighting with excessive iron oxides in the water supply but these are now reducing, but I'm sure will still be present.
Anybody willing to offer an opinion please?

Grrrrrr photo rejected again! it's a 1Meg .jpg!!!! I hate this function.....
Okay deep breath.
It a large green leaf one of the older leaves with two yellowing areas 4cm2 in the middle of the leafy on each side of the cenral vein The plant is growing emerged from a tank with two strip LED desk lamps providing light

My favourite website for plant deficiencies is this one:
http://www.theplantedtank.co.uk/deficiencies.htm

Based on your description, I would suggest :
"Magnesium: [shows on] Old [leaves as] Yellow patches with veins remaining green. Similar to iron as it prevents plant from absorbing iron. Distorted leaf growth."

Hi Andy,

Not sure if this post will be too late now that the 120 day deadline has passed. If it is published, what was the outcome of the question you posed?

JPC