Our filter also leaked at the bottom drain plug but only when the filter was running. If I put it on too tight it would leak and too loose it would leak. Finally we found a perfect place, not too tight, not too loose and it stopped leaking but our water level never decreased.
From looking at the pictures, I wonder if the filter is cracked at the bottom? I would start when the filter is running feel the top of the filter and the sides and see if it's wet, then you can rule out the gaskets at the top. Feel around the pump all fittings to see if anything is wet. You may have to take the sand out of the filter to check if it's cracked at the bottom, you don't want to be tilt the filter with sand in it because it can damage the laterals. I learned that the hard way. I put my sand filter in the garage every year and from tilting it back to put it on the dolly I cracked the stand that the laterals screw into.
I feel for you, water leaks in general drive me crazy.
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