I too was going to ask the same question.
I have fishes in mind (red parrot chiclid, 2 rock shrimps, 2 guppies and a harlequin fish).
I don't want to harm the fish by adding flourite or anything that can harm it, but my substrate right now is playsand with gravel on top. w/ co2 injection and the plants seem to be doing really well. . . . But i wonder for longevity if i should add flourite or as Valiko was wondering ... laterite.
"Is the "ice cube" method the only way to add Laterite into the substrate?"
What is the ice cube method?