Although this was previously recommended, canaries and pet finches do not need insoluble grit to help them grind up and digest their food. This is because they remove the husks from their seeds before swallowing and therefore is very little indigestible material in their diet - even though some websites still recommend adding it to your birds' cages.
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.