I like Dave's advice.
There are many catfish that can fit the bill. So far your preferences are:
-- ok with a featherfin syno
-- like Pimelodus pictus but larger
-- visible / not a hider
-- ok with hard water (do you know the number? how hard are we talking? if you don't know, it would be beneficial if you took a sample of your water into a local fish shop because they can measure carbonate hardness KH and general hardness GH for you for free)
-- it'd be good to know pH (again an LFS can do it for free)