New to ponds, old owners had koi. They are all dead

[Faebinder]

The Lilly pads are in the kiddie pool so they can try to survive. Light green plant is on a free floating piece

Yeah so your pond depends on the walls and floating algae to eat up the ammonia. The filter helps convert ammonia to nitrites then nitrates. My advice stands.. no koi, best just get goldfish. There are awesome ones out there.

Sanke Goldfish

 

[SarahT]

It's a type of uv filtration system, it's built into the ground but has a pump in the pond to pump water into it. The only things I was able to find was a video in Chinese or so

Your pond is very nice looking and some goldfish swimming around will be fun to watch. There's some work to do now, but once that's done, maintenance doesn't have to be overwhelming.
The UV light in that kind of filter should be replaced every year, even if the bulb looks like it's working. Were there any directions from the previous owners about cleaning the UV filter? Many of those filters have filter pads inside them that need to be flushed regularly to take out the muck they've accumulated.
At some point fairly soon, you will want to replace the corrugated hose to/from the filter, pond pump, and waterfall with flex PVC or spa hose. The corrugated hose has a tendency to split and leak. If you can, raise the pump in the pond a foot off the bottom of the pond so if there's a hose or other failure, the pond doesn't drain completely.
Longer term, consider adding a "bog"/ upflow wetland filtration area, to your pond. There is lots of discussion on this Forum about bogs. I added one last year, replacing a filter similar to yours and am pleased with how clear the water is and how comparatively easy-care it is.
I'm not the best of water lily growers, but they probably could do just fine in the pond now, not the kiddie pool. I didn't catch the reason they're in the pool not the pond. There are a number of threads on this forum about planting and growing water lilies.

 

[poconojoe]

My pond is over 12 years old and I never do water changes or clean the liner....ever.
The only thing I do is slowly run my net along the bottom in the Spring and Fall to scoop up any debris that may have collected down there.

 

[addy1]

My pond is over 12 years old and I never do water changes or clean the liner....ever.
The only thing I do is slowly run my net along the bottom in the Spring and Fall to scoop up any debris that may have collected down there.

Me either, I don't even net the bottom, used to but never pulled enough stuff out to make it worth while.

 

[GBBUDD]

Don’t be too tough on yourself…. There are people with much larger ponds and expertise that still lose fish… it’s a learning process and even when everything is right, things still happen. So sorry for your loss

Part of life the gift it comes to end . As they say eat right, exercise , get plenty of sleep, never get married, die any ways,