When you go away on vacation, I think the best thing to do is to have somebody that you trust, whether it's a neighbor or a friend, somebody who you can instruct on what to look for and what to do in your absence, come and check on the tank everyday.
You usually only have ten to 15 hours to spot and fix a problem if there's a problem with the pump or there's a power outage, anything more than that, your fish will start to die because they're going to run out of oxygen. That being said, you also want somebody to feed the fish everyday. You can go ever other day with the feedings, but if there is a problem, you would then benefit from having a friend or somebody check on the tank everyday. If that isn't a luxury you can afford or have, then you can get an automatic feeder or one of the little dissolving pyramid feeders for freshwater. If you do go with the latter, the little dissolving pyramids, they need the water chemistry to be such so that they dissolve at the rate that releases the food in a consistent frequency that doesn't overtax the tank. So the pH needs to be within a certain range and the temperature needs to be within a certain range, so that it dissolves within a seven day period or a three day period depending on which one you buy, so it releases the food for the fish in the right amount.
Automatic feeders are great too. They're battery operated. They mount on the top of the tank and they release the food once or twice a day depending on the setting. They feed a dry or pellet food. So for saltwater tanks where you feed frozen foods, the only way to do this is to have somebody feed the fish for you. If the fish are used to eating frozen foods, usually switching to dry foods just for vacation doesn't work that well. If you don't have an automatic feeder and it's a saltwater tank and you can't go with a dissolving feeder and you don't have anyone to come check on the tank, usually fish are fine three to five days, no problem without food. In fact, before the fish are shipped into this country, they are typically not fed for three or four days, so that they can be packed in small bags, so that their own waste doesn't pollute the bag of water. So they can go three, four, five days with no food, after that they're going to need to eat. They're going to start to get a little thin, they can start chasing each other around, and it's just downright cruel to not feed a fish for that long. So I'm not condoning that behavior, but if you're really in a bind or there's an emergency and you have to skip town, they can go three to five days without being fed. Just make sure there's enough water in the filter so that evaporation doesn't run the filter dry and the pump cavitates from low water levels. But the best is just to have somebody come.
Most people know at least one person they can trust to come and check on the tank, but don't have them do any maintenance, don't have them do any little cleanings, just have them feed. It's best to show them exactly how much to feed. Don't just say, "Feed a little," because people will abuse that and everybody interprets that differently and they'll probably overfeed.