One more thing, I noticed that your bottom drains are just something like glorified shower drain-caps (I mean the top cover, don't get offended please)..
Make a larger dome, easy with some concrete and using PVC 90' elbows as spacers to keep it about 3/4" off the floor. Seal it with a couple of water proof paint layers. My pond threat has some pictures on it.
That way you create much more flow over the bottom and clean out a much wider area than you'd would currently.
Cheers,
Luc
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