I have a heavily planted 12 gallon that I let go for about a year and all of the fish died but the plants and nerites thrived. I recently added fish to it but have a wierd problem that I have never had before. The fish that swim in the water column are happy and healthy but every scavenger I have put in it has died within a couple of days. I have added panda cories, albino cories, and a kuhli loach (not all at the same time and purchased from different LFS). Is there something that could have built up on the substrate? Before I got so sick, I had nanus cories that lived about 3 years and a sumo loach that lived for almost 6 years.
Substrate: eco complete
Plants: anubias nana, nana petite, crypt wendtii, madagascar lace, marimo ball
Fish: 4 black neon tetras, 6 white clouds, 1 dwarf gourami
Any ideas? I am stumped.