Fishbase.org shows 77-86 degrees. 68 degrees probably wouldn't kill them outright in seconds but long term they'd probably fade away. I certainly wouldn't "try it and see".
In the event of a power outage or something, they'd be ok around that temp. for a little while, but it wouldn't be good for them. That's the kind of situation that can lead to ich.
There's no reason to keep them that cold. Just get a heater and keep them at 78-84. I think warm stable temps for clown loaches not only make their life easier, but yours too.