The goal is to mark fish in a way that the mark would only be seen under infrared light. The tag should resist to water for some days (at least one) and should not be toxic. advance.
I'd try using an AEInvisible ink through dermojet marker. Maybe AE6330 Ink. These inks are made for use on skin and are water proof for about 24hrs maybe more. I have never tried it on fish. I have used internal tags and external markings like fin or gill plate punches. But all depends on species, purpose, and how many fish you need to be able to identify again (each individual, a group, several groups) because then the markers need to be more detailed the more individual markings required.
Good luck and I'd like to hear if you find a good method to do this.
I used Biophotonic marks from New West Technologies in my PhD studies (about early 2000's). This marks are microspheres of latex (natural rubber) applied by tattoo, wich has high biocompatibility, high retention, and they are visible at UV light and so on. I used it for over 27 months and I did recapture marked fishs with this time.
Look for these products in New West technologies: www.newwesttechnologies.com
P.S.: I have attached a photo of a specimen of Rhamdia quelen (Ostariophysi: Heptapteridae) maked with this method.
I was looking for a signal that would be visible under infrared light. An additional requirement is that emission should be in the infrared range (because of camera filter). Sigma sell some pigment which fluoresce in different wavelength. I'll pursue my quest !
Shachaf Ben Ezra well it didn't work as expected and I didn't find alternative solution at this time and didn't look recently. Sorry not to be able to be more helpful.