Update: Apparently I have poor observational skills. Or the tank water was murkier than I realized before changing yesterday. I now see that all of them have these folds in their skin except for the larger one.

I have made a new observation. Some of them have dark bumps on their underside, 2 or 3 that are the same color as their backs, while the rest of the belly is a light gray. These are definitely new developments within the last week or so. What are these?