Maybe there is a buildup of air in the drain pipe so its not draining at its full capacity until all the air gets flushed out? Did you try plugging it in, waiting till it almost overflows, unplugging it, waiting for it to drain a little, then plugging it back in again, etc etc? That should eventually flush all of the air out of the system.
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32"x32"x20" Cube-ish tank
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