The male end of a "standard" water hose will wedge just inside my gravity water fill port - no adapter needed. I have around 60# city water pressure, and the hose stays put with the tap open fully.
I get about a 2' fountain squirting back out when full... then I turn off the tap and wait until the fountain stops. At that point, I turn the tap back on low - just to be sure it's full to the brim... which it usually is.
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