I wrote about this on another thread, but my neighbor has a small pond -- 6 x 8 maybe? Probably 2 feet deep max, maybe less. It had a little fountain to move the water and no skimmer. I bought my home in '98 and the pond was there before that. A while back, the current homeowner did a big renovation. They turned off all the power for that and it stayed off to the pond for over a year. I was surprised to see everything looking beautiful. After all those years, I think the pond just has a perfect little biological balance going between fish and plants. No mosquitos. Clear water. Pretty neat!
So in some small cases, we have been sold a bill of goods, Jim Baker was selling "Prayer Kits" to poor old ladies.
Sure, in a large pond with a lot of koi making a lot of waste, the balance of the nitrogen cycle must be correct.
But, on a smaller scale if you simply want Goldfish and a few plants we don't need to go to the bank and take out a loan to have a nice little pond that has a waterfall when we want it and the entire fish population won't die if we lose power for three weeks. There are natural ponds not far from our home and I can scrounge some plants there, they may not be lillies but will add beauty and oxygen.
It will be a challenge in our part of Mexico but it will be worth it.