Emperors can do hard water?? I like the penguins too! My water is "makes mollies happy" hard though, 253ppm if I remember rightly. Us hardwater folks are basically restricted to livebearers, cichlids or rainbowfish though. I do like mollies and guppies, endlers; and a friend tracked down cories that can cope with harder water (bronzes, sterbai and schwartzi, among a few others), and I'm really getting into psuedomugli. I'm not really into cichlids on the whole though.I’ve always had soft water, and I get jealous sometimes of people with hard water. I’ve missed out on a lot of CA cichlids and livebearers.
There’s a few tetras that are good in hard water... Penguins (T. boehlkei), Emperors, and Silver-tips come to mind.
So my larger tanks I keep hardwater, and have a 15.5g that I soften with rainwater so I can keep pygmy cories and otos in there, so only small soft water fish since I can't/won't deal with the expense and hassle of RO, and can only really collect enough rainwater to sustain a 15.5g.
I didn't know about those tetra though! If you know of any other fish that are surprisingly good with harder water other than multis, please do let me know!