fish care when you are away
Let's say you are on vacation for 3 weeks. What do you do about your fish when you are away for some reason? You have several options.
Ask your local fish store if they will keep your fish
They might be able to keep the fish for 3 weeks. Most local fish stores get requests like this all the time. At worst, they will mix your fish in with their other similar fish, and you'll have to start from scratch (tank that needs to get cycled again, new fish). But your fish will be okay.
Ask a friend or two friends to come over to feed them on schedule
Ask a friend to come every other day to feed your fish. They'll be fine that way - most fish don't need to be fed daily. You could make two friends alternate, and that way each will come in only once every four days. At three weeks, you're stretching the limit on water quality, but most fish will be okay. Any longer, and you should teach your friend on how to change water.
Setup an automatic feeder
Setup an automatic feeder (costs around $30), 
which usually lasts about a week before the food gets all gooey and the feeder malfunctions. Teach a friend how to clean and set it up. That way, your friend only has to come in twice (you set it before you go, your friend comes in 7 days later, then 14 days later, and you're back on the 21st day).
Setup multiple feeders
The most expensive option. Buy three feeders (one for each week) and set them up in advance. Your friend just comes and swaps feeders. One downside is that food loses nutritional value when exposed to air - but it's only 3 weeks, so you should be okay.
Move your tank to a friend's house
Most painful option. Transport fishes and tank (separately) to a friends house. What's problematic is that you have to take most of the water too. Take a big plastic tub there in advance. Do one run to get some of the water there, and the fishes, then runs to get most of the water there, then the tank. Unfortunately, you have to do this again once you get back.
Food blocks are often not a good option
Petstores sell vacation food blocks for fishtanks (e.g. Aqua Pharmaceuticals' 7 day pyramind fish-feeder, costs about $20), with differing varieties for different types of fish. The basic principle is that the block dissolves in the water over a period of time, releasing small bits that the fish can nibble on over a week or so. The idea is brilliant, but the science hasn't quite matched it. The blocks release enough dissolved matter into the water to allow for algae and bacterial blooms. People frequently return to find a cloudy, or even black water, tank. Sometimes, it can cause an ammonia spike as too much uneaten food material saturates the tank. If push comes to shove, you could try a food block - but be aware that it risks your fish's health and may require work to stabilize your tank again.