my amazon sword which is about 8 month old, was dowing very will, but am noticing that the new leaves are starting to melt, CO2 IS ON TARGET, am doing EI, could that be a calcium issue? or iron issue?
I'm certainly not as qualified as some of the other members here...but assuming your CO2 and nutrient levels are sufficient, could this be a lighting issue? When was the last time you changed your bulbs?
When you say the CO2 is on target what do you mean? Are you using a drop checker with 4 dKH distilled or deioniized water in it? Or is the "target" the pH you set on a controller. One thing large sword plants do is slurp up nutrients like a sponge. So, you might want to try increasing your nitrate and phosphate dosing by a factor of 1 1/2.
The EI method doesn't use testing to determine how much of any nutrient to dose. When you decided to cut back to half the EI recommended amounts you stopped using the EI method. With 4 watts per gallon on any tank bigger than 10-15 gallons is very high light intensity, which will make the plants grow as fast as they can, limited by whichever nutrient they run out of first. I suggest raising that light higher above the water to reduce the intensity, or using fewer of the light bulbs. Then go back to dosing per EI.