kribs

Kribs don't need and really do not prefer brackish water. They can handle pHs of less than 6 up to 8 though (they're quite adaptive).

As for your tank, I'd give it a try in a 55, but keep an eye on things.

I just got 4 albino kribs about 2 weeks ago.
I have a pH of 6.5-6.7 (kept there with CO2), a kh of 4, and a gh of 2-3. The colors that are starting to show are awesome. I do not have salt in my water at all. So far they are not quarelling with any of my other fish. But all of my fish are very peaceful and small. You can see some pics in my sig.