just converting a pool into a Koi pond, is not really effective. Koi fish produce a lot of waste. And only way to have an effective filter system is to have bottom drains.
Then also on a standard pool, the slope of the floor is not enough, so leaves that fell into the pool, and even the fish waste will to float down close to a bottom drain. It should be 30 degree or more angles to be really effective. That also forces you to use multiple bottom drains, spaces over the bottom of the pool. So no, it will not work.
Using 50mm pipes on the bottom drains, will also not work. They will block up, as the flow speed is not enough. Must be at least 110mm pipe.
Another thing not taken into account on Koi ponds. The filter system should be gravity fed. Water flow out of pond, via filter system to last chamber where the return pump must be. Should not use a pump in the bottom of the drain to pump water to the filter system. Why smash up the leaves into tiny bits with pump impeller, then try to take that particles out with your filtration. Its ok on smaller ponds maybe, less that 2square meter surface. But still, not a good idea to use pumps between pond and filter, but rather on return between filter and pump.
Even with gravity fed systems, and using 110mm bottom drain pipes, the system can still block up. So how do you flush the bottom of the pool? Must use standpipes inside a weir. That is a deep pit between pond and filter. So you suck the weir dry, pull the standpipe, and the empty weir gets filled rapidly by strong flow from one bottom drain.
So you see, there is one hell of a lot of changes you need to do to be able to convert a swimming pool into an effective koi pond.
OK, as this is a marine forum, what must be done to convert the pool into a marine environment, with enough water flow for corals, preventing detritus, keeping rainwater and other groundwater out, nutrient removal,.... Damn, its a complete rebuild to convert it into a Koi pond, never mind even contemplating a marine rock pool.