Yeah, neon, 6 of the same species is the "minimum" for happy corys. It's not that they don't get along but are much more active with several of the same. They may even spawn for you!
Thanks, tl. I hope that barbel erosion will never be a problem for my fish again. I asked about "your" natural products because without knowing what is likely wrong it might be a good first step
I agree there are several possible causes for barbel wear/erosion as the article I saved said. High nitrate, flukes, bacteria. Did you notice all the articles cited were 19+ years old?
I think part of the big gravel issue is that it holds a large amount of debris (ie "crap") & also it's hard for corys to dig down for foods but they try. I freaked out once or twice on seeing my sidthimunki loaches digging for live worms with just their tails showing. They were fine & on Eco Complete, a smallish rounded volcanic gravel (not the Eco sand). Many people say that's not a "loach/catfish appropriate" substrate but I say BS, my sewellia breed on it although their barbels are short anyway. Too fine a substrate can have possible compaction issues, as I think you've mentioned, with anaerobic pockets.