I don't think the manatees did much swimming in there... more like just laying around in water and muck...
Definitely puzzling that this was a reality not even all that long ago.
And while it is now used as tortoise terrarium, I seem to remember from my visits to Artis some years ago that it was also for a while used as an aquarium with African fish.
@KevinB true about the fish. After the last manatee - the 3,5 year old Bertus, the son of Joop and a female I don't know the name of - moved to Randers in 2002, some cichlids were kept in this bassin. After a period with cane toads, it's now inhabited by tortoises.