It doesn't stop, it just gets worse, you always want to go bigger!
You mentioned you built a brick wall around it before, have you ever thought of building a raised brick built pond, would make it easy to make different sections which could cascade into each other.
Also, in response to kazs_angel. Putting gravel in the pond is not a good idea in my opinion. It's another object for the fish to catch themselves on (don't know how much your fish add up too but when your putting hundreds/thousand of pounds worth of koi in the pond it's the last thing you want). Also, silt will settle in the gravel and the gravel will make it awkward to clean. You want as little obstruction in the pond as possible for muck to settle.
Haven't been on here in ages! Been really busy and sort of dropped out the fishkeeping hobby for a while, but I'm back into it now & recently just purchased 4 new Koi for the pond.
There is also talk of an extension on the cards
Not happy with the size of my pond (7ft x 4ft x 4ft) so the plan is to leave length 7ft but extend the width to 10ft, and build up 16". New section will also be dug down 1ft so I will have a 6ft x 7ft shelf which will be 28" deep and a 7ft x 4ft deep section at 5.5ft deep
Attached is a pic of what my pond looks like now & what I want to do
And here is a little video of my fish qualities not the best it was a spare of the moment thing on my mobile phone...
The filter will smell yes because this is where all the muck is trapped. You can/should clean it out every so often using bucketed pond water, NOT a hosepipe.
That's a lot of fish for a pond that size, depending on their size you could be close to overstocking.
When the water temperature drops, i.e winter the fish become less active and don't have as much as an appetite as they do in the summer months. You should be feeding your fish as much as they will consume within 5 minutes, twice a day in the summer then cut this down to only one time a day during the colder months.
Providing your pond isn't indoors in a shed etc the fish would most likely survive without being fed because of natural foods that fall into the water, but still it is adviseable to feed them.
Re: Thousands of tiny black insects in pipework.
Sound like larvae, which are completely harmless to your fish & feed on the debris in the water, so if anything they are doing your pond some good. Ever wondered why swarms of flies shoot out when you open the lid?