I would prefer a per fish tracking but cannot think of a good way to ID them at such a small size.
They make numbered ID tags. I think the most common type are called "floy tags".
I do NOT see our experts using them on the forum - but they may use them in some limited circumstances. You might get an expert opinion if you start a new thread on that topic. (If you really want to do individual tracking.)
Floy Tag Catalog
