Common Mistakes When Starting a New Aquarium

Overfeeding and water filtration are two things to keep an eye on

Freshwater aquarium
Daniel Caja / Getty Images

Beginning hobbyist fish keepers new to establishing an aquarium should read up on the mistakes of people who came before them and avoid some common pitfalls.

Starting Too Small

With the availability of mini-aquarium packages, it may be appealing to go small. However, for beginners, choosing a small aquarium is courting failure. When the water volume is small, key water parameters change very quickly and leave no room for error.

Even experienced aquarists are challenged by a small aquarium. Newcomers to the hobby should stay away from tanks under 20 gallons until you become experienced. 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. The water in a new tank needs to stabilize. Gases are dissolved in the water as well as minerals, heavy metals, and chemicals added to local water treatment facilities. The water itself can harm the fish. Aquarium water needs to be treated with water conditioner to neutralize harmful materials and allowed to stand for a day or so to allow dissolved gases to escape and the pH to stabilize. You want to make sure the filtration system is working and the heater brings the water to the correct temperature, and the tank is not leaking for at least a day. Then, it will be safe to introduce a few fish to the aquarium.

Adding Too Many Fish at Once

You may feel eager to fill the tank with fish, but unfortunately adding too many fish all at once is another common mistake of new owners. Until the beneficial bacterial colonies have become fully established in the bio-filter, the aquarium cannot safely support a full load of fish. In the beginning, only add a couple of small hardy fish. Wait until both the ammonia and nitrite levels have risen and then fallen to zero before adding more fish. It usually take about 3-6 weeks for a new aquarium to go through the initial Nitrogen cycle, so fish should be added only a few per week during this time.

Overstocking the Aquarium

It is very common for new owners to overstock the aquarium. Although an experienced person may successfully keep a school of 20 small fish in a ten-gallon aquarium, it would be disastrous for a beginner to attempt it.

The net gallons of water should be the amount of water actually placed in the aquarium after the gravel and decorations are in it. You will want to use an 80 percent ratio of tank volume to actual water in the aquarium.

For example, a "10-gallon aquarium" may only hold 8 gallons of water after the decorations and gravel have been added. Using the one inch of fish length per gallon of water rule, 8 inches of fish is a maximum number to be safely kept. That could be 8 fish that grow to be one inch long when full grown, or 4 fish that grow to be 2-inches long when full grown. It is always wise to go under the maximum to rather than over. This is just a general rule and bigger aquariums with large filtration systems can often hold more fish than this, if the water quality is managed properly.

Keeping Incompatible Fish

New aquarium owners often choose fish that look appealing to them without knowing their environmental needs. Some fish may fight with one another or require widely different water conditions. Always research each species before choosing tank mates. Select peaceful fish that thrive in similar water conditions.

Overfeeding Your Fish

The number one mistake made by fish owners is overfeeding the fish. Fish are opportunistic and will seek food at all times. Just because they appear hungry does not mean they need to be fed all the time. Feed them no more than they completely consume in five minutes. if food is left over after five minutes, remove the food with a net and feed less food next time.

During startup, feed fish no more than once per day; 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. Once your aquarium has cycled and the ammonia and nitrite levels are zero, you can start feeding your fish twice daily.

Insufficient Filtration

The flow of water through the filter is what makes the water safe for your fish. An aquarium filter should pass all the water in the tank through it at least three times per hour. If it does not, it is too small. If in doubt about filter size, move to the next larger size. You cannot over-filter, but you can definitely under-filter, and the results can be harmful to your fish.

illustration of common mistakes to avoid with your aquarium
Illustration: © The Spruce, 2019

Not Testing the Water

New owners do not magically have full knowledge of the nitrogen cycle nor do they know they need to test the water chemistry in their aquarium. As a result, they can fail to take steps to deal with harmful toxins that accumulate in the water quickly in a new aquarium.

When the tank is first set up, allow it to run for a day or two. Before adding the fish, test the pH, hardness, alkalinity, ammonia, nitrite and nitrate levels for a baseline record. During the startup cycle, it is important to test the ammonia and nitrite often (see nitrogen cycle for details). Once the tank is well established, test the water monthly to find unseen problems that may be brewing. If fish suddenly die, test the water to see if anything has changed.

Not Changing the Water

New owners are not always educated about 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 water and replacing it with fresh water. Normally a partial water change is made weekly in a new aquarium, and then once a month once the bio-filter has become established. Removing and replacing about 20% of the water is usually sufficient with each water change. Make more frequent water changes if the water quality test results are not at the correct levels for your fish. And always add dechlorinator or water conditioner to the tap water before adding it into your aquarium.

Although your fish may not die if you fail at 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.