Among sub-sect of Shias, the mandatory taxes which includes zakat, is called dasond, and 20% of the collected amount is set aside as income for the Imams.
Some require certain minimum nisab for zakatability, some don't.
Other differences between Islamic scholars on zakat and nisab are acknowledged as follows by Yusuf al-Qaradawi, Unlike prayers, we observe that even the ratio, the exemption, the kinds of wealth that are zakatable are subject to differences among scholars.