Depends really, does it already contain perlite? I always like to add perlite, but I've never used Fox Farm. I like about 25-30% perlite. I also always add Garden Lime, it keeps PH in check. That's just my mix tho, this is one of those touchy subjects that every grower needs to find for themselves. There are a ton of other positive things that you could throw in, but I'm not sure if you'd need any in Fox Farm, supposedly that stuff kicks ass.