Veronika
I don't know much about Clivias but I see the others have already giving you very useful tips.
What I wanted to say is that you might reconsider the climate zone in your info box which says zone 9b which is almost tropical! You could just plant your Clivia outside.
I think zone 6 would be more appropriate for Slovakia..
the climate is relatively continental with almost no extremes below minimal -20°C (-4°F) or above maximal +37°C (+99°F).