I still have no clue what causes the yellowing.
I kept the one in the red pot for more than a year in very good condition. Then I repotted to semi-hydro this March and put it in the window by mistake. Then bottom leaves were turning yellow. I thought it was beacuse of too much light and moved it away from the window. Yellowing hasn't stopped though. I was browsing the web and books and forums and all I found was either too much water or nutrient deficiency. Tried to adjust both, no result, every now and then a bottom leaf or two turn yellow and die.
I have also had problems with whiteflies. Only on this plant they always returned.

Then in the middle of June the second plant came (white pot). I salvaged it from clearence but was in good condition. And it also has yellowing bottom leaves. It was never put in direct light, it is not in hydroculture and is on different watering schedule.

It is just so very annoying that I can't find out what is their problem! They are both developing well, bringing out new leaves on top, even at the bottom of the stem but those turn yellow and wilt after a while.
Please if anyone knows what causes this, let me know.

MsGF