I find it easier for sure! The bigger the tank (or pond), the more forgiving it tends to be because it doesn’t get dirty as fast or dangerous as fast so you don’t have to do as many water changes. I actually haven’t even really done any. Just top it off when it needs it but the rain has done a lot too. I do clean it and I’m a bit behind lately but it’s been pretty easy.
I find inside fish tend to be a lot of work. It’s why I don’t have any anymore. I got some bettas in November after not having any for years but they died this spring and I never got any more. I was going to but at the time I had 2 separate tanks for them (and a big bowl briefly for a third one who died before I could move him

) plus the goldfish tank so it was a lot


now that the goldfish are outside it’s easier but I don’t like inside tanks. Too much work.
I think if I ever did get another one I would get a huge one like a 29, 55, or a 75, something where I wouldn’t be doing a lot of water changes as often. Bigger tanks are actually surprisingly easier lol they don’t get unsafe as fast. I want a huge tropical community tank eventually.
Oh and goldfish are extremely messy so she may just need a bigger filter!!! I’m not sure if koi are as messy, I think they might be cleaner, but idk. What other fish does she have? Might be the combination of them maybe? I would try a bigger filter though. Or rinsing off the filter pad every so often. They usually need like double the recommended amount though haha but thankfully they are pretty forgiving fish.
And thanks!