Labeling Frag Plug help!

[atomic081]

Hey all, I have a Zoanthid grow out system of about 40 different types and it can get hard to remember all the names. I had written on the back of the 4x4 travertine tiles they are mounted to with a sharpie and let them dry before putting them in the tank. The problem is the writing has started to come off. Before I cant read them, I would like a way to permanently label the tiles. Anyone have any recommendation?

 

[Suohhen]

you could use a dry erase or something else outside the tank but inside the only thing I can think of that would be quick and effective would be a labor maker but I doubt the labels are safe.