Our first pond was something like 5000 gallons, earth bottomed, with a
14*3 by one-to-three foot deep swampy-type filter pond. It was low
maintenance until my wife decided that we needed to edge it in limestone
boulders and the sides all fell in.

The next one was much smaller and had a liner. It was low maintenance
until we had to fill it in to sell the house.. That was a lot of work.