I'm not sure that is the type of fuse you want to be using, but obviously it did blow so there's something to be said for that. The fuse is on the output side of the solar charge controller, correct? If it was on the output side, your solar charge controller should not have seen any power when the PV panels weren't producing any watts (at night).
The 20 amp output of the solar charge controller requires a fuse better than 20 amps. We usually explain this as, "avoiding nuisance trips." 1.25 times the expected amperage is the recommendation.