Hi,
Baby koi can last in a tub, especially since you will have a pump in it. You can get some feeders for the fish. Automatic ones or try the goldfish ones they have at Petstores.

Get some goldfish, they spawn like crazy, and what the koi or goldfish don't eat, you can have more babies to grow out to supply your pond. Koi take longer to spawn, at least that has been my experience or they just ate their eggs before I knew about it.

Buy the feeder goldfish and soon they will be bigger and will start making babies for you. For the koi to get bigger they really need nutrition or supplementation.j

Good luck. Also maybe a pond club in your area might have some extra koi that are the UGLY kind. lol

How did you get predatory fish in the pond? Is it connected to natural waterways?

It is illegal to introduce exotic (non-native) fish such as koi and goldfish to wild waterways.

I purchased an auto feeder. Goldfish will never get large enough to avoid pickeral and the other pred fish. Birds, like ducks, heron and kingfishers inadvertantly bring various fish eggs with them from pond to stream to ponds.
BTW, my big pond is about two acres. All kinds of critters in it. Above mentioned birds, along with migrating ducks, geese, occassional sighting of rare green herons. Beaver, mink, muscrat, racoon, bear, turtles, ect. I love it.
As far as 'wild water4ways'? I agree. My pond has a flow stream which is gated. I talked to the same DEC guy about koi, that I received the trout and triploid grass carp from. As long as they are contained properly.

I sometimes find goldfish fry in my Skippy biofilter. They actually have to go through my pump and find their way up through 150 gallons of filter media. I would think your gates would have to filter very finely to keep all fry from escaping.

But of course you can do whatever is legal.

BTW, it must be wonderful having a 2-acre pond! I'm in heaven with my 7000-gallon one.

Good luck.

Sharon