Stop diesel venting when filling tank?

Boat is a Carver 530 Voyager and with two vents on either side. When I fill
(even using a normal slow pump) I get diesel fuel venting. Yesterday I had
to add 200 gal. to each side and venting started after I'd put in < 10 gal.
I'm at a lose to understand why this is happening. Foaming seems unlikely as
it would need to have filled 50% of the tank!

Any ideas of what is going on? Or even better a simple remedy that works!

Thanks,

Dave


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