I can't really take credit for proposing the idea that filters aren't needed to keep a pond. The idea has been around for hundreds of years. It's only recently that the internet has allowed for legions of newly minted "experts" that suddenly all ponds must have filters. And the more filters there are for people to push the more filters are seemed to be required.
I was first introduced to these concepts by people like
Greg Bickal back when he was just getting started.
Roddy Conrad who even to this day will push a person to really study and understand a subject even when he isn't initially right or blasted for questioning dogma.
Norm Meck who's theories on algae from many years ago are still ignored by virtually all "experts" even though his theories are very easy to test. And a few others. So that's were I got the idea.
Crystal clear water 100% of the time does take some effort. I assume that's what you mean by not "ugly looking". I think a stream is the most effective tool. I was first turned on to this idea by I think Greg Bickal, who had bright green new pond water and added a stream and the water clearer within a few days. He was surpised, it wasn't the goal. The pond forum experts stuck to their guns and said the stream had nothing to do with the clearing, that it was a nutrient thing that the algae had been starved even though nutrient levels were easily tested. Quacks don't mind ignoring facts.
It may have just been a coincidence that Greg's pond cleared. The algea's life cycle was probably right at a tipping point and the stream allow Norm Meck's bacteria to get the upper hand. Ponds with streams can certainly be green. If you run a UV filter for just a week or two that also can tip the balance of power into the bacteria's favor. Couple that with a stream and the green water killing bacteria can keep the upper hand for years, possibly forever.
It is of course possible that a "bio filter" might provide the green water killing bacteria an upper hand, but it seems marginal, not very effective. In pond X sure. But for a majority of ponds, probably not. Green algea love sunlight but can be killed by UV, whether from a filter or sunlight. A UV filter is just way more effective. The thin sheet of water in a stream exposed to sun is not a happy place for algae. And it brings them into close proximity to the killer bacteria, bad for algae, good for the bacteria which get a convey belt of food. The same process happens inside a "bio filter", just doesn't seem to be as effective. Could also just be a size issue.
A gravel bog can also be a bad place for algae. The killer bacteria are happy there and, if large enough, an algae cell could be stuck in there, in the dark, for hours, even days. So they can go from having 8 hours of sunlight per day to just a couple. That reduces their ability to reproduce and the killer bacteria can get the upper hand.
That can still leave suspended bits of dead algae in the water. Most people would accept that as crystal clear water. A fabric filter can normally take clear to the next level, as can water changes.
So I would use filters to clear ponds, but only for short periods of time and only for specific needs. Nothing permanent. Certainly no "bio filters". I never came close to having a fish load that required them. I found this by accident. In the beginning I was always experimenting with bio filters, measuring ammonia, and realized ammonia was 0 whether I had a bio filter or not. I had just assumed I needed bio filters from reading internet pond forums.
As for muck...basically harmless or even good for fish by providing food. The only time I ever had an issue with muck is when there was a very heavy leaf load and after about 3 years clumps broke from the bottom and floated to the surface. That had to be cleaned.
Fish loads that push O2 limits are an entirely different story.
