We use a Clarke 41 litre per minute pump from Machine Mart. In our case the pump is higher than the IBC tank and we run the suction hose through the vent of the IBC filler (the vent has been removed - we have wrapped a cloth around it the prevent dirt getting back in. The pump is in a cupboard above the tank.) We also have a foot valve on the bottom of the suction hose in the IBC tank.
Some have carefully cut the top of their IBC tank and 'peeled' the plastic back to put a submersible pump inside. they have then 'sealed' the cut with silicone.
There is a picture of a diagram someone put up on here once, possibly a few years ago which will be worth doing a search for.
Edited: sorry I can't find the diagram that I was referring to, but someone cut an 'H' in the top of his ibc tank with his Stanley knife that was large enough to put his pump though.
Another idea was to make a diagonal cut through the tank opening and one down at 90 degrees to that (east to west and one cut toward south of a compass if you see what I mean.) This will allow you to open the filler size and put you sub pump in. It will then close back. The tank lid should hold it in place. and then seal the cuts with silicone.
Obviously you will have to cut other holes for the power cable and the outlet hose.
Spruce