Perhaps you might not need tags for your fish, but the folks at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) do. They've developed a way to 3D print titanium tags for tracking larger fish including Marlin and Tuna.
Titanium was chosen as it does not react to salty ocean water and can withstand the rigors of fish lifestyle.
The tags are embedded directly into the fish, and so require careful design to be effective. Previously the researchers were custom-milling individual tags, a more expensive process that takes much longer to complete. 3D printing permits quick turnaround of individualized fish tags.
Ocean fish, beware.