aquarium kid - this is iron deficiency. Pale new growth and a stopping of plant growth. The older leaves are brown due to diatoms which tend to show up in newly setup tanks.
Perhaps the soil you started with did not have a lot of iron to begin with. You'll have to add an iron supplement, Iron chelate 10% with DTPA is a good one which can be bought from: http://www.aquariumfertilizer.com/in...ditU=1&Regit=2
or you can use another iron supplement if you have one. Use about 0.2 ppm iron to help the plants recover. http://calc.petalphile.com/
can help you calculate how many grams, spoons, mL etc of any iron supplement you have to add to get 0.2 ppm.
Scroll down to Iron Deficiencies on my new DeficiencyFinder database site and you can see examples of other iron deficient species. Click on any of them for a more detailed description: http://deficiencyfinder.com/
Also, it you wouldn't mind and want to help me expand the database I could definitely use close up good quality (big) pictures of your iron deficient Ludwigia. Close up detailed shots of just one stem, along with some ludwigia next to some of your other plants would be great (showing that not all plants develop iron deficiency symptoms at the same time).
As you can see from the database I don't have many Ludwigia species yet and your photo would be very helpful. You can email the photos to me or PM me and I'll give you full credit for the photo (PM me for my email address).