Does it still get covered in muck, pulled into it by the suction?

Surface area gives muck time to break down, so there are always clear
spots. http://users.owt.com/jjspond/koipond/photos/set1/09.jpg I've used
one of these in the lily pond, opened it up to find a perfectly clean pump
within after a year of use. ~ jan

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(Do you know where your water quality is?)