Pygmaeus, hastatus, or habrosus cories could do. Possibly panda too. Not all of them together of course. What are the dimensions of the tank? Another one would be hara jerdoni/asian moth catfish.
Sorry I totally skipped over that your water is hard. Do you know the actual level of hardness? Also most fish can adapt, especially if you buy them locally they will likely already be used to the water.
It's easier for soft water fish to adapt to hard water than it is for hard water fish to adapt to soft water, I think.
If your tank was just a smidge bigger you could do red lizard catfish. I want some of these one day >.<
I imagine your best bet may be a small synodontis but I have no idea which one would be small enough for this tank. Sorry am not of much, if any, help!
*Edit* I found THIS but I am not sure how easy they are to find.