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I have tested my pond water when I was having terrible algae problems but it was just new pond syndrome. I do test every year to see if the hardness is going up because sometimes I have to fill it with tapwater instead of rainwater.
I also have done 0 water changes since we filled it. Its in the 2nd summer and all the plants & animals that use it seem happy.
The maintenance you do have to do is leaf removal if you have your pond in sun. If you don't you get massive algae blooms from all the nutrients released by the leaves & sometimes yellowish water if they contain a lot of tannins. Mine doesn't have a bacterial filter but I guess they need periodic cleanout to if you have one.