I have used dishwashing detergent and a few gallons of water
and gone for a sail on a windy day, then straight to the pump
Dishwashing detergent as it doesn't make many suds, and it's cheap
I don't understand why your outlet hose wouldn't be on the bottom, perhaps there is a sort of dip tube? Have to take the hose off and look I'm afraid, but that would tell you.