Fish Base has options for finding information in different languages, and even translates the whole site into Spanish by way of a mouse click on the "Languages" tab at the top of the window.
Fish Base in Spanish.
1. When you identify a fish in your fish guide that you want to know the Spanish name for, find the scientific name in the fish guide first.
2. Type it in to the appropriate field (Genus + Species) on the webpage I've linked to above, and then click "Search."
3. You will be taken to a page with all sorts of information about the fish. Amongst the initial information you will see a line called "Common Names" which lists a few and then invites you to learn
More. Click that "more."
4. You will now see a chart that gives lots of common names. Look in the country list to find the names used in Mexico. It's interesting to note that there may be several common names for the very same fish in different countries that share a common language or even in different parts of the same country.
For example, when I check in my own language, Portuguese, for a fish I know as Gray Angelfish and which is listed in the guides under the scientific name,
Pomacanthus arcuatus, I find the two common names I'm most familiar with in Portuguese, "Frade" and "Enxada," but I also find seventeen other common names listed.
Have fun!