My guess is that RES is like any other fish. If they don't have enough food, they will get stunted. The more you over stock them, the more likely that will happen. Perhaps you can make sure they have enough forage to do well. I know they will eat snails and worms. Perhaps others can give you all the other things they will eat.
I think you are right fish n chips. The eyes on these two RES (top two) have signs of stunting, they are overly large relative to their body size. I got a handful of these from the fish truck last year.
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