I have a small pond and I am not sure if I have to keep it spotless. It has a liner but on the sides, ( under the water) it is a bit slimey, I do not have anything on the bottom but it gets a little muckey which I scoop out weekly. The water is usually fairly clear unless I stir up the bottom.
no you dont have to keep it spotless as thats near impossible ,the slime is algae and is normal ,a uv filter would sort it out but if it isnt too bad i wouldnt worry,id try and add some sort of substrate ,gravel or something,you havent mentioned wether or not you have a pump/filter running these are essential for a healthy fishpond otherwise the water will stagnate ,check the water parameters every few weeks and dont forget your partial water changes
