Any ideas about this detail? (Middle right rosette).

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In the thread You are not allowed to view links. Register or Login to view., I have noted that the "piscina" (fish pond) on the plan somehow reminds me the middle right rosette. Its fountain's double water-jet spouts, decorated as some two heads, reminds the two flows emerging from the central point of the rosette. I wondered, what if this two "fish tails" are the two streams of a fountain in a pond?

   

Piscina. Wikipedia:

The piscina is a Latin word originally applied to a You are not allowed to view links. Register or Login to view., and later used for natural or artificial pools for bathing, and also for a water tank or reservoir. In  ecclesiastical usage it was applied to the basin used for r ablutions and sometimes other sacraments.

Of course, it's hard to explain why an earthly usual fish pond with a fountain may look bubbled and nebulous. I see it as something allegorical, meaning some divine pond or vessel.
Surprising for myself, I've found interesting lines that exactly convey the impression from the image of the right middle rosette:
"The water you see here is from no source
that needs replenishment from cloudy vapors,
like streams that rise and fall: with constant force
it leaves a fountain that receives again
from God’s Will, every drop that it pours forth
to the two streams it sends across this plain.”
(Dante Alighieri. The Divine Comedy. Canto XVIII. Purgatory.)
The two streams in the verse - the rivers Lethe and Eunoe.