depends on species... most predetory fish do rather well on a fast... but species like cardinal tetras will die w/out food for that long. young fish also fair worse off then mature fish... and as ponted out some fish can and will turn on tank-mates when hungry... if you feel the need to have them fed.. have someone stop by mid point. have a bag of food already pre portioned ect... make it idiot proof.. and have them fed once while your gone. specific directions particularly if more then one tank, and a number they can reach you at if they have questions. we vacation every year for 1-2 weeks and this has been my stratagy for success... haveing someone fish savvy is good, but someone who understands how much you care about your pets imo is more important... they are going to call you if they notice any small thing, the "fish savvy" person may wait until it's to late, depending on how much they truely know.