Mine have been in everything from unheated water to my current temp of 26-27 (also a rio 180), but they're only in this heated tank because I have nowhere else to put them, and at five years old they're finally starting to die off - I have 3 left of 8 and I'm not setting up a coldwater tank just for them. I would actually like the temp lower in the 180 but the room temperature prevents that happening.
They do better in cooler water though, 28 is far too high for them.