Hi Helene
Yes – I’ve now seen your video and I think you’re more likely right with Paramecia (or something like them) rather than Ostracods. Your critters seem to be much more at the mercy of the tank’s current and occupy the whole water column ; while mine prefer to associate themselves with surfaces in their tank – although they do zip around when they feel like it!
Since it’s raining in Wellington (New Zealand) I did you a quick video of my Ostracods anyway! But probably nothing you hadn’t seen already during your earlier Googling. Here goes attempting the link …