Top 10 Mistakes Fish Owners Make

Adam Denish, DVM
By Adam Denish, DVM on Nov. 15, 2016
Image: bibiphoto / Shutterstock

Common Mistakes Fish Owners Make

By Adam Denish, DVM


Contrary to what many pet parents may believe, caring for and maintaining a happy, healthy home fish aquarium can require some legwork. From overfeeding your fish to using the wrong water in your tank, here are the top 10 mistakes new fish owners often make and how to avoid them:

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Overfeeding Your Fish

Overfeeding leads to overeating and excess waste, and a habit of overfeeding can lead to water quality issues that can make fish sick. Aim for feeding your fish the amount of food that it will consume in about three minutes. If you notice that too much fish food winds up trapped in the gravel, reduce the amount you are feeding. If you use flake food, refrain from tipping the bottle over the fish tank, instead take a small pinch with your fingers, or transfer food to a container with holes in the top for easier dispensing. If you do overfeed, remove the excess food in the tank with a net.


Most fish are opportunistic feeders and search for food all day long. Grazing herbivorous and young fish (like guppies or small angelfish) may need two small feedings per day and may benefit from having plants to nibble. Some fish are omnivorous and require more protein, like African Cichlids or, in saltwater aquariums, triggerfish. These fish can eat once per day and enjoy a more substantial meal of freeze dried or frozen worms or shrimp. Include scavengers like snails and crabs in your tank to balance out the community and clean up uneaten food. 

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Succumbing to New Tank Syndrome

“New tank syndrome” arises when a tank lacks a sufficient population of bacteria necessary to process the nitrogen compounds that result from fish waste.


Building up bacteria in your tank water starts by adding a few hardy, inexpensive fish (like tetras or platys to a freshwater aquarium or damsels to a saltwater aquarium) to your fish tank. After adding the starter fish, allow time for the nitrogen cycle to proceed. Some experts recommend adding sand or gravel from an established tank to move the process forward.


There are also a variety of additives that can help you establish the bacteria and cycle the water more quickly. Monitoring the nitrogen compounds with a chemical testing kit will reveal how the tank is cycling and indicate when you can begin slowly adding more fish. Most aquarium stores will gladly test water samples and offer guidance about the best practices with your tank. 

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Mismatching Your Species of Fish

Fish owners have lots to say about the personality of their pets. Before adding multiple species to your tank, connect with fellow fish hobbyists who can attest to whether two species will live together peacefully.


There are also charts available to help you mix and match species based on the level of aggression. If you happen to like an aggressive species of fish, such as Oscars or African cichlids, you will need to select tank mates that can tolerate aggression, usually others of the same species. One important exception is the betta fish, also known as Siamese fighting fish—and for good reason. This species should never be housed together. Two males put together will fight to the death. Even a male-female pairing should be under supervision and for the duration of breeding only, since they also will begin to fight.


Mixing aggressive fish with passive fish can lead to bullying. Fish that are too frightened to come out of hiding during feeding time will eventually starve to death. Avoid making impulsive purchases of fish and plan your tank selections in advance with research.

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Overcrowding Your Tank

Planning for your fish population is an important first step.


The rule to keep in mind is one gallon of water per inch of fish, though bear in mind that aggressive fish require more lateral swimming space. If you select aggressive fish, double the size of your fish in calculating the number of gallons of water per fish needed for swimming space.


Be sure to find out what the adult size of the fish you select will be and the habits of those fish. Also consider that rock work and decorations infringe on the swimming space and should be taken into account when determining adequate space for the inhabitants. 

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Taking a Vacation Without Back-Up

While you head off for a beach vacation, the care of your aquarium will need to be addressed. Properly training a pet-sitter to fill in for you while you are away is the best defense against disaster.


Make it as easy as possible for a sitter to give the correct amount of food by setting out portions in a weekly pill organizer. Explain how to check the temperature of the tank and make an adjustment if needed. Leave written instructions for how to deal with tank issues such as water evaporation, death of a fish or cloudy water conditions as well as contact information of a trusted aquarium store professional or fellow hobbyist.


If your vacation is for a short weekend, there are automatic feeders as well as slow dissolving food cakes available. Technology also makes it possible to have a camera focused on your aquarium for owners to check remotely and provide extra reassurance that everything is okay. 

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Not Understanding Temperature Control

Smaller tanks are subject to faster temperature swings than larger tanks. Use a simple temperature-sensitive sticker to help quickly monitor the heat. Fish and invertebrates are sensitive to shifts in temperature, so make it a point to check the heater or chiller regularly. Avoid placing the tank near a draft or a sunny window and monitor the temperature to stay between 72 and 75 degrees Fahrenheit.


Keep in mind that some species require slightly lower or higher temperatures.  For example, koi and their popular cousins, the goldfish, are best kept at temperatures between 50 and 70 degrees Fahrenheit, while freshwater angelfish come from warmer waters and prefer temperatures between 78 and 82 degrees Fahrenheit.   

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Overlooking Disease

Paying close attention to the appearance and habits of your fish can help you catch illness early. Early detection is the best way prevent disease from spreading to other members of your tank. Adding new fish is often the source of a disease. When fish are transferred to a new environment, the stress can weaken their immune system making the fish susceptible to infection.


Keeping a quarantine tank for newcomers to occupy before release into your main tank is an important step in guarding against the spread of disease. Most fish hobbyists advise that new fish should be quarantined for 21 to 28 days.


Fish should appear hardy, have clear skin and eat readily before transferring to your main tank. The quarantine tank also serves as a hospital when you have a fish that looks ill. If possible, transfer the fish to the quarantine tank and seek medical advice for the best treatment protocol. 

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Neglecting Your Tank

Forgetting to feed your fish, not keeping up with the maintenance of your tank and not monitoring its water quality will limit the lifespan of your aquarium. Successful fish hobbyists routinely maintain the needs of the tank on a schedule.


If marking the calendar to note when it’s time to change the filter or setting an alarm to feed the fish isn’t your style, consider some automatic feeders and water quality probes that might help keep you on track. There are even water quality monitoring devices that wirelessly connect to your home computer system to alert you of any changes.


If you fear your interest in the hobby is waning, take a trip to a public aquarium or even your local fish store to get some inspiration. 

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Being Impatient with Tank Maintenance

“Slow and steady wins the race.” 


This quote applies to tortoises racing overconfident hares as well as to fish hobbyists. Having a fully-stocked show tank does not happen overnight. Planning and researching the equipment, monitoring the water quality and purchasing compatible inhabitants are essential steps to success.


Taking the time to build up a show tank will help you avoid common mistakes and induct you into a long-lasting hobby that millions of people enjoy.

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Using “People” Water

The water you use for brushing your teeth and bathing is treated with chlorine and can be toxic to your fish. Treat water with chlorine-removing additives, following the directions for use.


Refrain from using any soaps on your aquarium equipment. Hot water and small measures of bleach carefully rinsed away should be sufficient. Store bottles of treated water to use for replenishing the tank after evaporation. Keep up with water changes every two or three weeks.