Can anyone clarify the difference between Melanoiden and Carapils? It's seems like they are used for adding body and head retention but Melanoiden is 20L and Carapils 2L.
Big difference.
Carapils/Carafoam is a dextrin malt, unfermentable by Saccharomyces. It only adds body, no aroma or flavor, and very little color.
Melanoiden Malt is very flavorful and aromatic. Besides for adding color (30-40L) it mimics the flavors produced during decoction mashing without going through the rigmarole. Sort of like a super Munich Malt, except melanoiden has no diastiatic power of its own. It's very useful in many recipes.
Both need to be mashed.