Canaries and other pet finches will always remove the husks from the seeds that they are eating, because of this there is little indigestible material in their diets.For this reason, they do not need the extra grit to grind up and digest their food, though some websites may recommend that you add it to your birds’ cage.
Neither Red-throated Parrotfinches or any other species of pet finch need soluble grit
Like every other bird, a finches stomach has two compartments, this includes a small section called the gizzard. The gizzard is what grinds up the food that the birds are eating. Almost all budgerigar seed mixes will also come with small bits of soluble grit, most of the time oyster shell is used. The oyster shell is then dissolved and turns into a source of calcium for the birds. A finch's calcium needs should be fulfilled via a cuttlefish bone and a mineral fortified seed mix so grit won’t be needed.
If a bird eats too much grit it can die of a blocked crop. In general, soluble grit should be avoided as it provides almost no rela benefit and can even cause problems If you are giving your birds soluble grit, make sure that it is always in very small pieces. Make sure to check your mineral bloc, if this provides the birds any source of calcium whatsoever, it is preferable to the grit.