Dear Moreno

I have been a experienced with hard coral tagging also for coral grown and mortality marking, which I have used small plastic plate attached by  emboss label using emboss type machine  that many available at book store maybe.  This tagging could be  hold out for 2-3 years in underwater...

I agree with Jérome Mallefet, better putting a tag to the hard substratum at the base, depending on the conditions it may be used a two component adhesive glue that works well underwater. I attach a sample of how the growth can be controlled with photographs, the measurement can be assured by putting a metric strip into the frame.

Many thanks for your responses. I appreciate it.

Mr,Trainito, do you have any work published regarding corals' growth rates? 

Very nice pictures by the way, almost a decadal work visiting the same colonies. 

Kind Regards

Thank you for your appreciation, I have not published any work about this argument, the images and other observations are the result of a personal deepening about the collection of visual data on a long time span. I have shared them with researchers of Università Politecnica delle Marche , as a basis to develope a project of citizen science. 

A thin colored wire, at the same time take a picture for later comparison 

Nailing an endurable plastic or aluminium  tag on the rocky bottom close to the base of the octocoral, at a distance that avoid brushing the tissue of the colony.

We have used plastic tags nailed and cemented in the substrate (rock) for a gorgonian's monitoring since December 2013 in Qatar. Still in good condition! Sorry but no autorisation from our Customer to publish the results. If you need help for some projects, don't hesitate!