i know it is supposed to be a slow blow fuse because the guy at coralvue told me. i had a new style 250 watt ballast that was not working, and that was the first thing he told me to check. the first fuses i tried to use were fast blow, and they burned immediately. when i called david ( the guy at coralvue), he told me to use slow blow. my ballast burned those as well, so coralvue replaced it with the new style ballast. the people at coralvue were very nice and responsive. the only issue i had was with getting in touch with them, because this was a month or so ago when they were moving.