Weird sized biocube 29 plumbing?

[borillion]

So I picked up one of these random flow generators from Vivid Creative Aquatics, and the slip fit adapter is just a bit too small for my outlet.

Taking the existing plumbing there I'm measuring about 20.9 - 21 mm (13/16 in ) in diameter on the inner slip fit that came installed with the tank.

Anybody familiar with this? Bit unexpected.

 

[borillion]

ROFL turns out I have a Biocube 32 ( forgot ) and this entire time though it was a 29 due to displacement.