The reason he said to have somebody else take care of the fish is because your tank is not cycled yet. When a tank is not cycled, there are no bacteria to break down the ammonia in the tank, and ammonia is toxic to fish. Basically a cycle goes through several stages. First you need ammonia. Once you have ammonia, certain bacteria will grow and you will have nitrite. As you start getting nitrite, bacteria continues to grow and you get nitrate. Once you get nitrates, the nitrites and ammonia levels go down. The bacteria fights both of those off. An example of a cycled tanks water parameters are Ammonia-0ppm, Nitrite- 0ppm, Nitrate 5-20ppm. Right now, you probably have some ammonia (if its had a chance to build up), and no nitrite or nitrate. The ammonia will slowly poison the fish to death. Cycling a tank with fish in it will take much longer than without due to having to do water changes almost daily to keep ammonia levels tollerable to fish. Without the higher ammonia levels (4-5ppm), the bacteria growth is slowed down, resulting in a longer cycling period. Without fish, most cycles take about 2 weeks. If you had a cycled tank, you could simply pull the filter media from that and use it to seed the new tank, resulting in a MUCH faster cycle. Does that make more sense? I know its not exactly technically correct, but thats the jist of it