Okay, what about this https://www.versele-laga.com/en/vl/prestige/products/prestige-gritwithcoralSand isn't ideal for a shell/calcium grit (sand is not water soluble). I would try and find one that is completely shell.
Sand would be suitable to birds that don't shell their seeds, like chickens, ducks and doves. Since zebra finches shell their seeds they do not need sand.