There are many things that can cause your fish to become ill, or die, and most are preventable if you know some basics. Below we will try to cover some of the main contributing factors related to fish death and illness.
1. AGGRESSION: Not all fish are alike, and if you try to mix the wrong fish together, you will soon find this out. Some fish are predators and will eat smaller fish, some fish just like to fight. While this is generally more of a problem with African and South/Central American cichlids, it can still be problematic with other fish. We list aggression levels on each of our fish; these would generally be the aggression level inside their particular group of fish (ie. Africans/South American/catfish, etc). If in doubt, PLEASE ASK. We can give you plenty of information as to which fish can go with each other.
2. NEW TANK SYNDROME: When first starting a new tank, there's not any good bacteria established in the tank to consume the harmful ammonia that the fish will create from feces/urine. Without this good bacteria the ammonia will quickly build to toxic levels, which can VERY quickly kill your fish. The good bacteria will begin to form, but it can only move so fast. If you put too many fish in a tank too soon, the ammonia will run unchecked and likely kill many of your fish. Some people like to do a tank cycle with just a few hardy fish. This will usually take about a month, during which your ammonia will slowly spike, followed by nitrite, and finally nitrate. All the while, the good bacteria will be forming. Once this "process" is complete, your ammonia and nitrite will be down to 0, and you will only have nitrate readings. Nitrate is removed during your water changes.
There are other means of expediting the cycling of a tank - see our FAQ for more info.
3. LACK OF OXYGEN: fish need oxygen to breathe just as much as we do. At any given time your tank will have a certain amount of oxygen; if this amount falls below the required amount needed by your fish, then they will begin to have difficulty breathing, you will see them at the top of the tank gasping for air, and if not corrected they will eventually suffocate and die. Oxygen is brought into the water through the surface of the water interacting with the air. The more you can agitate and turn over your water (through filtration), the more oxygen you are going to get into the tank. Powerheads and wave makers are also great for creating more oxygen exchange in your aquarium.