Ok, one, I'm not a newbie to koi. True koi keepers stock lighty so their fish will get over 3 feet long. Your tank is overstocked and you are going to end up stunting your fish. In my pond, I had two 2' channel cats in it and the pond was 600 gallons and it seemed overstocked. Lets think about this, 250 gallons for a 3 foot fish. That doesn't seem cramped to you. Were all of your friends Koi under a foot long? They were probalby stunted because 5' x 5' x 2 feet deep isn't even big enough for two adult koi that have reached their true size, not a foot long. I haven't even gotten around to the fact that you have a bass and a catfish in there too. Did you know that bass grow really fast, so do channel cats. Channel cats grow 1 pound for every 2 pounds of food they consume. They grow really fast so you will be over stocked in that 60 very, very soon. So please, before you go and call me a newbie, go read up on Koi. They aren't made for tanks.
And pond filters only break down ammonia and nitrite, in a heavily stocked pond like your friends you better have lots of plants or you are going to have lots of nitrates and green water.
My pond, the one with the channel cats in it, has clear water.
I don't mean to be mean, but I'm not trying to be a no it all, but a 60 is way too small for those fish and you are going to have problems soon if you do not move them into a pond in the next month or two.