I’ve never used a clarifier when I brew. However a friend used one of those Mangrove Jack kit packs for his first time. I noticed it comes with a packet of beer clarifier. On inspection I noted this seems to be no more than gelatine(?). Essentially 5g (18oz) per 23L (6gal). Does that really work?
I don’t have specific experience with Mangrove Jacks’ process, but, yes gelatin works very well in clearing beer. I have used gelatin for years in my beer. Although measurements vary a bit, mix approximately 1 tsp of unflavored powdered gelatin to 1 cup of water heated to 150F, too much hotter and you’ll make jello (not the goal here). The gelatin will work best when your beer is cold, so chilling your beer prior to mixing the gelatin is a good idea. Once your beer is chilled, gently pour the gelatin mixture in the beer, a gentle stirring with a sanitized spoon will complete the process. Now, keeping the beer chilled, allow it to sit for a few days, generally I usually give it a week and you should see that your beer has cleared. A very gooey trub will settle at the bottom of your fermenter. Once cleared, rack the clear beer into your keg or bottling bucket and go from there.