I think a lot more info on your pond neds to be given. The first is how is your pond filled? Frogs are ok with koi, but don't expect any fry. Bullfrogs will eat anything they can get into their mouths. I had one snatch a baby duckling I added to my pond for my grandkids sake. The darn frog had it within minutes of putting him and three others. Fortunately I was able to net them out and rescue the poor thing. Needless to say we were all stunned by that. Filtration on your pond that size would best be answered by other folks with more knowledge on the subject. My pond averages 3 feet and is about 12' by 15'. I got into contact with a KHA koi health advisor from one of the koi clubs in our area. He sure straightened me out on the right way to filter my pond.
I had spent a couple of thousand dollars on uv lights, magnets, undersized filters from our local dealer. It was only this past season that I had clear water after having built my pond thirteen years ago. That would be my suggestion. Find a local koi club and ask if they have a KHA. There is no charge and the info is priceless. The guy was very passionate about sharing information to us less or misinformed koi ponders.