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It's pretty normal to tip $5 per tank in Mexico. That should be divided up by the crew, though you can give each one something if you like. I have given a bit more or less, depending on the service. Also, usually it's at the end of the week if we have the same crew all week. Otherwise, daily if you don't know. At least in Cozumel, I am happy to pay it, given the service received, which is normally excellent.
Is that $5 per tank per day ? ... so for a 5 day 2 tank per day trip ... you are talking about $50