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[Netflix Witcher] What happened to the mage students who got turned into eels by Tissaia de Vries?

What was the motivation to turn students into eels? Is being turned into an eel a known risk of attending mage school? Were those who were turned into eels willing and informed participants of eel conversion? How do other mages trust Tissaia de Vries and view her as a good person knowing she turned students into eels?
gpt
From ScreenRant:  


The apprentices actually serve a purpose as eels, as cruel as this transformation (and the whole “ascend” trick) is. By turning them into eels, Tissaia is taking away their control, but not their power, thus serving as a conduit for Aretuza. Yennefer wasn’t chosen as Tissaia saw her potential, and turning her into an eel would have been a real waste. The official site for Netflix’s The Witcher explains this a bit better, although briefly, in its description of Aretuza. It explains that the pool with white water at the bottom of the tower is Aretuza’s power source, and the “mages not capable of harnessing chaos are transformed into eels, forced to live out their days in this pool and provide power to the academy.”
  

  
The criteria for choosing which apprentices will be turned into eels and which ones will have proper initiations (and thus become sorceresses) seems to be the least talented girls in the group, though there might be a talented one among them from time to time, chosen to serve as an eel as she can’t be trusted. Tissaia mentioned to Yennefer that their powers make them “conduits of chaos” and that there are two types of mages – those who control their emotions and those who let their emotions control them – so the eels’ main purpose is to help them to better control that chaos and turn it into magic. It’s a weird practice, but it wasn’t a random act from Tissaia, though it is quite cruel.