Depends on what kind of fish you have... Both are safe for all fish, but S is smaller.
If I remember right, Grade S is a little smaller than poppy seeds and looks like sand.
Grade T is about 4-5 times the size, more irregularly shaped (but not sharp), and still very nice.
Would you prefer something more sand-like, then go with S.
If not, go with T.
Do be sure to rinse it extremely thoroughly in hot water. It's totally inert, but there is a TON of dust.
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