Thanks Bruce! I would venture to say that it's at least 4' deep and that's why you have some fairly large catfish in there. Cool! And that's a good size pump and I don't blame you for not pumping 24/7 because it can get costly.
With the kind of Koi I was keeping (Nishikigoi) and the cost of each, I had no choice but to run the pump 24/7. I was into a lot of KOI shows and even became a judge. Whether it's fish, horses, dogs, orchids or whatever ... when you get into competing them, you have to spend money getting good quality stuff ... or else you're not even in the ball game.
Koi or should I say "Nishikigoi" shows especially in Japan is a very big deal and they raise the best Koi North east of there called "Niigata" where the soil seems to be special for raising these things. Many breeders try to duplicate the same in the U.S. but never can come close.
I know what you mean about sinking in the mud and finding a tree branch to pull yourself out. Happened to me here in Hawaii while I was kayaking one of the rivers. I was using a very small kayak when someone in a motor boat flew by and dumped me into the river. Although the water is only about 8' deep, the bottom was silth and muddy. I came close to drowning, because every time I tried pulling myself back up on the kayak, it would roll over.
Tired as I was, I manged to use the kayak to swim where there were hanging branches and pulled myself back up. That happened about 8 years ago and I was already 66 years old! I guess my time wasn't up yet! LOL!