That is completely inaccurate. I'm not trying to put down a forum regular btw. Just trying to state my experiences. I have had my red claw crab with a betta and a pleco for months on end now. From the very first day, the betta and the crab would stare at each other (betta being the initiator) and then the betta would just leave. It's of course a true matter of the specific crab you get. A lot of fish are community fish but that doesn't mean you can't get a fish that is a bully. Same goes for the crab.
I have had no problems at all with the my two fish though and in fact my crab just molt last night!
With that being said, it is possible for the red claw crab to be in freshwater only. I understand that brackish is their preference but my crab wouldn't be molting if he weren't growing and healthy either. Hope this helps
