Maybe it was addressed but I didn't see it but how much debris is n the bottom better yet how much muck? If your cleaning filters every two days or so it tells me that there is a lot debris in the pond it also suggests that the current filter pads are beyond viable and need replaced. Another problem I see is that u added koi to a 500 gallon pond not just a couple but 10 that is a big no no in such a small pond you are over populated w fish and I mean with the waste they produce. People don't seem to realize that koi will grow to max even in a small pond and the amount of waste they produce is crazy. One full size koi produces as much waste as 8-9 goldfish so u can do the math. Plants are a must in any pond and chemicals are a very last resort. Most chemicals don't even work and they r just sales gimmicks to produce a buck.
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