hassansin - your plants have an iron deficiency. New white/pale green growth which eventually dies.
Though you have Hygro. corymbosa in your tank the closest species I have in the database to it is Hygro. polysperma, they react in similar ways to a lack of iron. You can read more about it here:
http://deficiencyfinder.com/?page_id=549
You are going to need to add iron to the tank, either by liquid fertilizer products (SeaChem's stuff etc) or using DTPA or EDTA chelated iron from:
http://www.aquariumfertilizer.com/index.asp?Option1=cats&Edit=2&EditU=1&Regit=2 or another site that sells it. Use this site:
http://calc.petalphile.com/ to calculate how much of whatever iron fertilizer you choose to add to the tank. You will need to add 0.2 ppm two or three times a week to stop the plants from deteriorating further.
Also, it would be extremely helpful if you were able to take a few more photos of the white tips of the plants, before you add the iron to the tank. Then in a week or so take another set of photos from the same angle to compare the change. Your plants will respond quickly to the iron.
Also, your tank looks a bit dim, you might want to consider getting a new light for the future. Plants rely on light to make all the energy they need and no amount of fertilizing will keep them growing healthily and fast without enough light. This is why your lower leaves are dropping off.