Thanks everyone.
@Jessie6162 many thanks for that link I will print it and check more carefully the leaves.
@sooby, I have to check again because I can't remember if the leaves are in the center or not, now it's almost midnight and I can't see well outside. The last picture is one leaf on Stella de Oro, it was not the only one, I took off three and every one of them had white points similar to the second picture on tips. From the picture it's not clear but the last piece of the leaves (the piece coming out of the fan) is totally white. I read that chlorosis can stop chlorophyll production thus leaves becoming white.
The soil is almost all like clay, when I plant and some time in the year I add common garden soil, but basically it's clay, and for what I know it's full of earthworms down there (I didn't check recently, but I took of a bit of soil for a small pot and weeks after I found earthworms in the pot).
I will try with the fertilizer, I never fertilize the DLs and this summer I watered a lot (compared to the past year) with tap water, full of limescale (I filter it for drinking but I can't for watering).
