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3) I have two spare pumps (Danner Supreme Classic 9.5 and Rio+ 2100) but not sure how to use them for vacuuming either. It seems like the debris would ruin the pump and don't know what type of hoses to fit the connections either, since I use normal garden hoses to fill and empty the tank. Also, these pumps need to run underwater so that is another complicating factor.
I would just gravel vac and drain the tank separately. Gravel vac using gravity into a bucket/tub. Then, use a pump to pump water out to a drain or outside.
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