My Spanish isn't great, but I think it would be like ar-coh-ee-ris. I'm not sure where to place the stress.
EDIT: Found a forvo link.
http://www.forvo.com/word/trucha_arcoiris/#es In the link, the woman seems to be pronouncing it like this: ar-coh-EE-rees.
This message was edited 2/26/2012, 10:04 AM