I have a small galvanized tank pond, about 150 gallons, that has been going a few years now. I have 3-5 goldfish in it that started out as 3 for a dollar feeder fish. At some point they had babies because I saw some in colors I didn't buy, but now I'm back down to three 3-4" fish.
I have dwarf cattail, creeping jenny, feathery marestail, yellow nymphoides, and a bonus water plantain growing, plus a sad water hyacinth the goldfish like to munch. The pump is just a small one from a bog box store in a crate with filter 'batting' and bioballs, but it's the plants that do most of the work and my water has kept clear & tests well even in the sun. It's above ground but has mounds of calendula shading most of it, plus a light shade cloth on a bamboo frame when it's really hot. Some of the plants are on full & half concrete blocks with slate on top and the fish seem to like using them as caves.
The outflow is from an old well spout and puts a fair bit of oxygen in the water, but I'm about to change it to a diff fountain head the hummingbirds can access better.
I leave it running all winter, adding a 250 watt de icer when it gets really cold (I'm near the Canadian border).
Everything seems healthy, and I do a weekly or so 25% water change using a spigot I installed that flows to a boggy area, then top up with regular hose water. I didn't seal the tank at all, just hosed it out, filled it up & added water conditioner.
It's added a ton of wildlife to my garden and inspired me to build a bigger pond when I can afford it.