I ran with this until 20/20.
My opinion is that it needs to be increased from 3 to 5 meter radius, which is the size of a 12/12 Sigil of consumption.
At the current size, it is very rare to get creatures in it for long, unless you are tanking a single creature in place. Otherwise it procs all over the place, which imho waters it down to a very weak proc.
Any slight move will drag the creature out of the effect, much like a small Blackwater Cocktail - and typically only one mob fits in the radius. Unlike BWC, though, you can not simply reapply this often. SoC size seems reasonable to get a few enemies.
TLDR: increasing the radius will reduce the overly-large rng associated with the proc.
As a comparison, I was running Flame Torrent (T1) at the same time on a secondary skill, and that was constantly in close play; even with only a 50% chaos–>vit conversion, it was far far more impactful than Fetid Pool, due to it’s consistent and controllable proc location relative to the character.
Sometimes Fetid procs on a different floor…it just depends which attack trips the proc. Very often there are a few pools outside of the main battle spot. It is very clunky to have to pull the mobs over to your tiny and spread-out pools. It is most often not worth the effort for the limited damage or risk.
Also:
I suggest increasing the constellation rewards to double or more. 1 yellow and 1 purple is a pretty meagre incentive to complete the 7 point Affliction constellation. It is a tier 2 and as such should be a nice stepping stone to tier 3, not a cul de sac (dead end street).