After a year of doubt thank you for helping me to make conclusion that my zelkova is chinese elm.
You're welcome. I have seen this misidentification so many times now that when I see a thread about Zelkova I immediately get suspicious. Here are two more links that should offer some more insight.
Airconditioner was a very good point, I have it but almost never use it as I considerate it unhealthy but remind me that someone here few weeks ago switch it on 13 C degree and left it for 4hours. Do you think maybe the bonsai received a winter signal??
It is not a 'winter signal' that concerns me as much as the lack of moisture in the air. Chinese Elms will hold their leaves late in the season, even well into winter, so I don't imagine a few cooler hours effected it at all.
Dry air is much more of a problem for all trees kept indoors and running air conditioning makes the situation worse. Low humidity indoors, in general, can very well be a part of your problem, but again, I doubt a few hours is the issue here.
Do you think that the natural mineral watter might be good for bonsai or?
I water all of my trees with well water that is rather high in mineral content and don't have a problem. I almost always water from above though, which helps to flush out any accumulated salts or minerals.
Winters here can be very cold during the day -5 C and the nights even -12 so I think its very riscy unless I make some bunker for it.
An acclimated Chinese Elm is able to overwinter without issue although some precautions must be taken. By acclimated I mean that it must be allowed to remain outside from about midsummer on. Dormancy is a process that begins after the summer solstice with the shortening of days. The tree should experience progressively cooler nights and frosts. Even a few good freezes will not harm it providing temps don't get too low.
By the time winter really sets in I move mine into an unheated garage that is attached to my home. In this way it stays slightly above freezing for most of the winter. If we get a really cold period it will again freeze, even in the garage, but will be fine as temperatures never really get extreme in this location. To recap, freezing is OK if acclimated but protect it from really low temperatures and cold, drying winds. This means that your balcony is going to be a problem.
But wait, all of this applies to a healthy tree entering dormancy outside. What you do right now is more of an emergency situation. Unfortunately I don't have any great insights to offer you. For now try this, snip off the end of a shoot or two and look for green, living tissue. Start with just the tip and see what you find. If there is no sign of life try cutting back just a little fatrther. Also, with your fingernail just barely nick the bark on the trunk and look for the same green. If you get past this step with positive results you may be able to save it. I await your response.