Even though that RTC appears to be in a fairly small tank he looks to be in great condition and the h2o looks nice and clean...kudos to this guy to at least put in the work to maintain a suitable home...hopefully he upgrades soon.
Kind of sad for the rat...its cruel to feed that live...and it probably isnt the best for his fish either. Frozen would be a better idea all around.
I dont think that eating a rat will harm him at all. Unless it was poisoned, or caught in his throat, there shouldnt be an issue.
If he is still trying to eat, that is always a good sign.