Weird... Any reason for the twice a day changes? Do you notice this issue when you do that or did it start around the double dip in changes?
Have you tested the water after only the 12 hour age as opposed to the 24 hr? Is something off there maybe?
The only thing that comes to my mind in reading your questionnaire (thank you for doing so) is something with the extra WC. I know its way more common in wilds, but some have said, that too big large or frequent changes can have a negative effect or bring on "whirling" like behavior which it seems you are experiencing or seeing. Have you tried to take a day off or give them a break and lower feedings a bit and see how he behaves?