Brown spots on plants

[Olga]

Hello! I have a fresh water aquarium that is 6 months old. I am trying to grow some plants, but they are not doing well so far. There is new growth, but leaves develop brown spots after a while and die. I add liquid CO2 every day and Flourish by Seachem 2 times a week. Here are pictures of what the leaves look like before they die. Any idea of what is wrong with my nutrient balance? Thank you.

 

[SwampeastMike]

How soft is the water? Leaves as transparent as those in the top picture likely lack calcium and/or magnesium.