The only fish in the dirt-y tank is a common pleco, no other fish. It was used as a tank to sink some driftwood and the common pleco didn't suit my 60g when I added the BNP's. The junk RCS are fine in there as well with the population still growing. Those I trap and give to the LFS for them to sell but I think the employees keep taking them home. lol
I'm guessing all that stuff on top of the sand is a combination of waste, rotten wood and the avocado leafs I add every now and then.
Last I checked the gh and kh were way up there and I assume that's from the crushed coral.