Starting Too Small
With the availability of mini-aquarium packages it's become appealing to go small. However, for beginners, choosing a small aquarium is courting failure. Why? Because when the water volume is small, key water parameters change very quickly, leaving no room for error.
Even experienced aquarists are challenged by a small aquarium. Newcomers to the hobby, stay away from tanks under 20 gallons until they've gotten some experience under their belt. Remember, the bigger the tank, the less impact a mistake will have on the fish.
Adding Fish Too Soon
New aquarium owners are eager to add fish, often the same day they set up the tank. Some are lucky but many will quickly lose some, or all, of their fish. What went wrong? The water in a new tank hasn't stabilized yet. Gases are dissolved in the water, as well as minerals, heavy metals, and chemicals added by local water treatment facilities.
Without going into lengthy detail about water chemistry, suffice it to say that dissolved constituents in the water can harm the fish. Aquarium water should be treated to neutralize harmful materials, and allowed to stand for a day or so to allow dissolved gases to escape and the pH to stabilize. Only then is it safe to introduce fish to the aquarium.
Adding Too Many Fish at Once
The fish owner isn't eager to fill the tank with fish? Unfortunately adding too many fish all at once is another common new owner mistake. Until the bacterial colonies have fully established, the aquarium cannot safely support a full load of fish. Only add a couple of small hardy fish initially. Wait until both the ammonia and nitrite levels have risen, and then fallen to zero, before adding more fish.
Even getting through the initial startup, it's very common for new owners to overstock the aquarium. Although an experienced person may successfully keep a school of twenty small fish in a ten gallon aquarium, it would be disastrous for a beginner to attempt it.
Debate exists over the inch per gallon rule, but it provides a good basic yardstick from which to start. I recommend taking eighty percent of the net gallons of water in the tank as the maximum number of inches of fish to keep in the tank. The net gallons of water is the amount of water actually placed in the aquarium after the gravel and decorations a in it.
For example, lets say an aquarium holds 16 gallons of water after the decorations and gravel have been added. Multiplying a 16 times 80% yields a result of 12.8 - or about 13 inches of fish as a maximum number. It is always wise to go under the maximum to rather than all over.
Keeping Incompatible Fish
New aquarium owners often choose fish that look appealing to them, without knowing the environmental needs of the fish. Some fish may fight with one another, or require widely different water conditions. Either way, they should not be kept together. Always research each species before choosing tank mates. Select peaceful fish that thrive in similar water conditions.
The number one mistake made by fish owners is overfeeding their fish. Fish are opportunistic and will seek food at all times. Just because they appear hungry, doesn't mean they need to be fed all the time. Feed them no more than is completely consumed in five minutes.
During startup feed fish no more than once per day, and during critical times when ammonia or nitrite levels are high, withhold feeding for a day or two to reduce the wastes being produced. Fish can easily go several days without food, and not suffer ill effects.
An aquarium filter should filter all the water in the tank through it at least three times per hour. If it doesn't, it is too small. If in doubt about filter size, move to the next size up. You can't over-filter, but you can definitely under-filter, and the results can be harmful to your fish.
Not Testing the Water
New owners aren't magically given full knowledge of the nitrogen cycle, and the need to monitor the water chemistry in their aquarium. As a result they often are unaware of the need to test their water, and fail to take steps to deal with harmful toxins.
When the tank is first set up, it should be allowed to run for a day or two. Before adding the fish the pH, hardness, ammonia, and nitrite levels should be tested for a baseline record. During the startup cycle it is important to test the ammonia and nitrites often (see Nitrogen Cycle for details). Once the tank is well established, test the water monthly to be aware of unseen problems that may be brewing. If fish suddenly die, test the water to see if anything has changed.
Not Changing the Water
On other area that new owners aren't always educated about is aquarium maintenance, which includes changing part of the water on a regular basis. Wastes build up in the tank that can only be removed by vacuuming the gravel and removing some of water and replacing it with fresh water.
Although your fish may not die if you fail to maintenance and regular water changes, they will be stressed by substandard water conditions. As a result they will be more susceptible to disease and often will have a shorter lifespan than they should have.