I don’t know much about garden ponds but we discovered we had one after I got rid of a lot of nettles and brambles around the bottom of the garden one afternoon after we moved in. A few weeks later I cleared out 12 years of rotting leaves and gunge out of it which went straight onto the flower beds.
After cleaning the pond out I stuck a tarpaulin over it to stop water getting in, and was going to fill it in with soil a few months later. In that time, water filled up (how did that happen?) and taking the tarp up one day saw it was home to thousands of tadpoles! I couldn’t fill it in after that, could I?
It’s taken a few years to get sorted and because it costs a few bob to get electricity down the garden, there’s no fountain or flowing water to keep it fresh so it’s dependent on the oxygenating plants in there and the odd regular clean-out by yours truly. I also usually put a net over it in the autumn to stop the leaves getting in and that’s about all for maintenance.
It was looking well ropey a few months ago (above) so I bought a couple of bokashi mud balls off ebay for a few quid (no, I didn’t know what they were either before getting them.)
The pond is starting to look a bit better, you can even see the goldfish now (below) There’s also frogs, who love the slugs and snails in the garden (one of them is chilling out in the top RH corner of pic below), a couple of minnows and the odd newt. Not bad for something we didn’t even realise we had when we moved in. Expense is minimal, all plants were off ebay or from the garden centre/pet shop (and they multiply like billy-ho), the fish were cheap and the only cost now is the fish food which is a couple of quid a month. The only drawback is you have to watch out that you don’t get bitten by those pesky midges in the summer!