I have friends who have four little fish in a large bowl that has a tiny filter. I have told them in simple terms of the harm they are doing, but they now think their fish are "special" because the LFS said their setup would work well with their fish. Some of the "special" LFS instructions include cleaning the tiny filter every week with regular tap water, and replacing the filter every month.
I have had to stop talking about fish with them, and have just told them that if they ever start to get concerned about their fish I can put them in my pond.
One thing to keep in mind is that in a big pond the ratio of water volume to fish is probably much larger than a tank which means the fish don't have such a dramatic and quick impact on the overall water quality. If you are putting a lot of big fish in a small pond, then reverse would be true.
A quite extreme example is when I had just three fry in a 400 liter pond. I didn't record any ammonia or nitrites ever, even though the fry were obviously eating and growing and getting bigger. Once I put the other 9 fish in, then I started getting readings but I think my filter had matured biologically by then.
Also, having a pond full of plants, bugs, snails, wildlife should all help with certain aspects of water quality.
As well as cycling the pond, it is just as important to make sure the tap water you add to the pond has been treated to remove the chlorine etc.
While I am no expert, it looks like a fungal problem. It has probably infected some wound or ulcer underneath. I hope others here can help further, but a google for goldfish fungus should get you advice for treatments etc.
I would say that it depends on the size of the pond and whether it has its own supply of water).
I have a small pond (850 liters) that is raised. I clean my filters by flushing pond water through them every week, and then I top up the pond with dechlorinated tap water (treated with Tetrapond Aquasafe). This replaces maybe 5% of the pond's water.
If it weren't for the need to clean the filters I doubt I would routinely change that much water, but then again I monitor the quality regularly so I have a feeling for how well it is doing.
Re: Black marks on scales on listless goldfish
The fish is still acting the same (not too active but swimming fine and eating fine) but has a lot more black on it now.
The blanket weed is finally starting to slow down and my nitrate readings are starting to register now, my latest reading is 0/0/5 Ammonia/Nitrite/Nitrate. Up until now I have had lots of 0/0/0 readings with massive blanket weed growth and I assume the blanket weed growth has been taking up most of the nitrate.
The water had a daytime temp of 12 degrees C today, and I am feeding the fish on wheatgerm already which they are eating as normal.
Here is a pic of the fish now, where you can see it is much more marked than before. (click on it for a high res pic)
I can't compare what I have to anything else, but my Oase Filtoclear 6000 and Aquamax Eco 6000 CWS came as a set and although they were expensive (~?400) my pond is looking great. It includes a UV filter and is very, very simple to clean.
My pond is only 850l and not overly stocked (8 young fish, no koi) so the filter is over rated for the job right now, but once the fish get bigger and we get a second generation of fish it will all help.
I have no experience of other filters etc and have only had this filter for around 8 months so I bow to others here who have far more experience than I do
Thanks BigKev, I have already taken your advice and used expanding foam to insulate parts of the gaps between wall and pond. It doesn't half make a mess, but thankfully its all hidden by the wall
And Mitchofmedway I have had a similar active spell in the pond too. The fish are thriving (apart from one that is starting to have black smudges along its flanks and rear) and the blanket weed has been growing like topsy. I hadn't thought of insulating the pipes doh!. Thats such an obvious thing to do, thanks.
Once the temperature drops below around 10 degrees you should switch to a wheatgerm food until it goes below around 6 degrees when you should stop altogether. Most of the stick and pellet food I have bought says something similar to this on it, so take a look at the packaging of anything you have for confirmation.
My pond is around 12 degrees during the day right now and the fish are eating far less than usual, so I am simply feeding them less (removing what they leave after ~20 minutes).
I have stopped feeding them treats (cucumber, peas, blood worm etc) now too, and I will wait now until the spring.
The fish will be far less active when it gets colder and I expect they will not come up for food so I won't bother feeding them then.