i would think that if the person doesn't have a tank themselves to know what to look for, that a "manual" might be a little overkill. i would make it no more than 1 page, in bold letters, quadruple spaced to just make sure the basics are covered...like water in the tank and food for the fish a few times a week.
IMO...
I think a lot would depend on how long you are away, how sophisticated your setup is and the brain power of your friends.
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I once had a Big tank...I now have two Huskies and a coyote