Koi pond without bottom drain

[mrsclem]

10X10X5.5 pond with 7 adult koi and 15-20 younger koi. No skimmer, no bottom drain and no plants in pond other than a few lilies hung from sides. Have window boxes used as mini bogs on sides of pond with water spraying from above. No problems with debris building up on bottom. Pond is netted for leaves. Savio filter, 55 gallon skippy and just added 150gallon stock tank bog. Bottom of the pond is clean enough to see folds in liner.