Here's my Grey Redhorse and bass story...
At work, one of the tanks I take care of and keep stocked is an 8ft long ~180 gal. run tank (there's a riffle tank and a pool tank too). The original members of the tank were a Guadalupe Bass (like a spotted), whatever sunnies could survive the wrath of the bass, 2 Channel Cats, and one Plecostomas born and bred in the San Marcos River (all fish over 12").
Anyway, last summer I happened to seine up three nice BIG Grey Redhorses. I'd had mixed luck with juvie redhorses, but decided to take a shot at the big ones. Originally, they were real spooky and banged all over the place when you looked at them, but with daily contact (and the occasional school group tearing through), they chilled out. I started them out on frozen blood worms and krill simce Greys are invertivores mostly. They took to the frozen food relatively quickly, and eventually I got them on algae wafers, shrimp pellets, and Omega One flakes w/frozen foods twice a week or so.
I was pretty proud of my big 'ol redhorses, and Christmas time came around, and being a state office, it was pretty quiet around there for a week or so. Needless to say, no-one fed the fish (I'm not going to drive 2 hrs round trip to feed fish)... The bass was apparently infuriated that no-one fed him and beat the tar out of the redhorses. He never touched the Cats. All Redhorses in the run were dead by the New Year. I almost killed that bass, and he's still on thin ice.
Well, that's my longwinded experience, hope it helps a little.
BTW-Now I have two 6 inchers and about a dozen 2" Redhorses in the riffle tank, but I'm not trying any more big ones in the run until I get a Juvie Guad Bass to put in there.