If its a pond which meets the requirements, then stick to filtering the exact amount of how much the pond actually holds. Instead of double filtering like you would for a tank or overstocked pond.
I give Koi at least 200 gallons each, and the Shubunkin should get 55 gallons. Then you can get a 30 gallon for the Ryukin. I actually recommend at least a 55G for him, which means you could provide him with a friend. But if you can't get that, 30 is good for him.
Let me know if you have any more questions!
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs