Ben,the 6" black "thing",is most likely a catfish,or,black bullhead.
It could also be a sculpin.
As to the range of "pollution" that these critters can tollerate,I have no idea.
Many of the older houses had septic systems,though a large percentage may now be on the munincipal sewer system.
There is a sizable leak in SOMEONES septic/sewer system,due to the wonderfull aroma around the first infeeder upstream from the Woodard entrance.
While large areas of the drainage are lightly developed,this is changing as I type! The only good news is that they will all be connected to Clarksville's sewage system. The bad news is that that sewage system discharges raw sewage 24-7 into the Red River,just upstream from the Cumberland River!
Given Clarksville's standards for anything,I seriously doubt that you will see any Troglobites in this cave 10 years from now.
Slide. Slide on the ice...