Probably just not getting enough light. I would just cut above the black parts, and replant the top.

 

It's not uncommon for stem plants to lose their lower leaves as they get taller. The light just doesn't get to them as needed down at the bottom. As Haeum said, just cut the tops and replant them. If you want bushiness let the tops reach the surface and watch how much branching they will take on. Then trim the top and replant and you'll have multi-branched stems coming up.

 

thks a lot. I'll work on your suggestions. I was expecting some deficiencies but I think I can log it as 'normal conditions', I'll work a bit on stems and take your suggestion to leave them to come to surface. Anyway, a very good plant to own to learn about your ferts.light.co2...etc..!

 

Another factor sometimes especially in light limited situations is how many stems you plant together. It's advisable to plant just a few (2 or 3) with twisters. Than grab another 2 or 3 and plant them together. You can still put them close together but more light will get to each stem as opposed to planting more or the whole bunch together.