Glad you got the rtc health back under control. Yes red tails can and most often will eat what can fit in their mouths but I think your biggest problem is going to be the bio load you’ve added, koi are extremely messy fish and rtc’s cause a huge bio load by themselves, keep checking the water pirimiters and see if your filter can cope but personally I’d get rid of the koi and keep the red tail solo unless you have a huge pond and filter system