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Setting Up a Freshwater Aquarium

Developed with and approved by a qualified veterinarian.

Freshwater Aquarium


Aquariums bring a level of relaxation and tranquility into the home. Freshwater aquariums start out as simple tanks of water into which aquatic life is placed with the goal of creating the ultimate aquatic environment. A little extra care and patience in the beginning will go a long way toward preventing headaches and creating a healthy and happy aquarium for years to come.

Setting up your freshwater aquarium

Choosing a location

Before any work is done with the tank itself, determine the best location for the aquarium. It should be placed away from direct sunlight, windows, outside doors, heat vents and air conditioners. Rapid changes in temperature are extremely stressful to aquatic life, and direct sunlight will quickly turn your beautiful tank into a murky green algae farm.

Also, keep in mind that water weighs about 8 pounds per gallon, so a 20-gallon aquarium will weigh 160+ pounds, and a 75-gallon aquarium will weigh a whopping 600+ pounds! Be sure your floor will support this amount of weight. Some rental agreements also limit the size of aquariums allowed. Check for nearby electrical outlets, as most freshwater tanks will need at least three sockets, depending on how much equipment the aquarium needs.

Setting up and preparing the tank

  • Start by rinsing out the aquarium with warm water to get rid of any dust or debris inside
  • Position the aquarium stand into place, making sure it is level. If you don't have access to a carpenter's level, place the tank on the stand and fill with 1–2 inches of water. Check that the water is an even distance from the top of the stand on all four sides; an unbalanced aquarium can be extremely hazardous. Placing a tank on an uneven or tilted surface increases the risk that the tank will tip over, crack or leak. A proper stand designed for an aquarium is important; using anything other than a manufactured tank stand may void your tank's warranty
  • Make sure there is enough space between the wall and the back of the aquarium to adequately fit filters and cords and allow easy access for maintenance
  • If using an aquarium background, affix your background to the tank before filling with water and placing the aquarium against the wall
  • Fill the tank about a third of the way. Carefully dry off the bottom edge of the tank and stand, then check for leaks. Look for water beading up on the bottom edge or running down the sides of the stand. If the aquarium leaks, empty it and return it to the store for a replacement. You can attempt to fix it, but repairing a leaky tank is difficult, with no guarantee of success

Building your aquarium 

  • Filter: Install the filter according to the manufacturer's directions. Do not plug your filter in yet
  • Substrate: Rinse the new substrate—a kitchen colander works well, but be sure to disinfect it before using it for food preparation—and décor. Place the substrate and all decorations into the aquarium. You can use a commercial nitrifying bacteria additive or 1–2 cupfuls of substrate from an existing aquarium; the existing substrate contains beneficial bacteria
  • Air stone: If you want bubbles, now is the time to hook up the air stone or action ornament, airline tubing, gang valve, check valve and air pump
  • Fill the tank: If no leaks have been noted, fill the tank the rest of the way with water. To protect the décor and aquascaping, as well as prevent splashing, place a small saucer or bowl into the tank and pour the water directly onto that. Check the tank for leaks
  • Water conditioner and supplements: Add the appropriate amount of water conditioner or additive to the aquarium per manufacturer’s instructions. Depending on the species of aquatic life you intend to add to your aquarium, you may need to add freshwater aquarium salt. This adds electrolytes, supports a healthy slime coat, aids in disease recovery and helps prevent parasites. Specific gravity in a freshwater tank should be between 1.003 and 1.004
  • Heater: Place the heater into the tank, but do not plug it in yet. The best placement depends on the type of heater. Non-submersible clip-on heaters that must hang vertically in the tank should be placed as close to the outflow of the filter as possible. Submersible heaters should be placed as close as possible to the intake of the filter. These placements allow the heated water to be better dispersed throughout the tank. Check the tank for leaks again
    • Wait at least 20 minutes to plug in the heater. This allows the internal thermometer to adjust to the water temperature, preventing damage to the heater. Follow the instructions included with the heater and adjust your tank to the appropriate temperature (usually around 72–82°F.)
  • Thermometer: Install the thermometer according to the manufacturer's instructions. The thermometer should be on the opposite end of the tank from the heater in a position that is easy to check. Place the hood and light (if applicable) onto the aquarium. Plug in the filter, light and air pump. Make sure the cords running from the tank touch the ground before looping back up to the plug. This is called a "drip loop" and prevents water from running down the cord into your electrical socket. You may find that the water level drops slightly when the filter starts. Add as much dechlorinated water as necessary to bring the water level to the correct level
How to set up a freshwater aquarium | Image

Stabilizing period

Phase one of your tank setup is now complete! Wait until the tank has remained stable for at least 24–48 hours before adding any fish. This allows the atmospheric gases in the water to dissipate and allows time for any needed temperature adjustments. The water may be cloudy. Wait until this cloudiness dissipates before adding aquatic life.

If the water temperature has remained stable for at least 24–48 hours, and the white cloudiness has cleared up, you are now ready to begin the cycling process of your new aquarium. Additives can help speed up the nitrogen cycle. Refer to the Nitrogen Cycle Care Sheet for your next steps.

Petco Pet Care Centers offer free water testing; we recommend bringing in a sample to ensure the water is safe for your aquatic life.

Ongoing Maintenance and Care

  • Maintaining great water quality with regular water changes and adequate filtration is important to help keep your freshwater tank healthy.
    • Daily: Check filter, water temperature and other equipment.
    • Weekly: Test water quality at least once a week. Scrape algae growth as needed.
    • Weekly to monthly: Change 10–25% of the total volume of water every 2–4 weeks, or as needed; change filter media monthly.
  • CAUTION: Never use soap or chemicals on any elements of a fish tank. A mixture of water and vinegar 50/50 works very well to clean the exterior of the aquarium and any components needing a refresh. Soap and other chemicals can be harmful to aquatic life



Tank Health/Troubleshooting

Health Issue Symptoms or Causes Suggested Action
Issue Cloudy water Causes Tank cycling (bacteria bloom), substrate residue, organics and heavy minerals in tap water Suggested Action Perform a 10–25% water change and siphon the substrate, removing residual debris; water clarifier additives may help; reverse osmosis water filters can remove contaminates found in tap water; add a nitrifying bacteria additive
Issue Green water Causes Algae bloom, too much light (direct sunlight), excessive nutrients, overfeeding, overstocked aquarium Action Reduce photo period and block any direct sunlight; increase water change frequency; reduce feeding; ultraviolet filtration can assist with destroying algae cells
Issue pH fluctuations Causes Buildup of organic material and debris, increased levels of CO2 Action Perform a 10–25% water change; reduce feeding; increase water circulation and surface agitation
Issue Ammonia spike Causes Insufficient nitrifying bacteria, too many fish Action Perform a 10–25% water change and do not siphon the substrate; reduce feeding; add a nitrifying bacteria additive



  • What supplies do you need for a freshwater fish aquarium? Some of the basic items you will need include an aquarium, stand, light, cover, heater, filter, thermometer, substrate, water conditioner, freshwater aquarium salt and décor.


Additional care sheets

Notes and sources

Ask a Pet Care Center store associate about Petco's selection of products available for the care and happiness of your new pet. All products carry a 100% money-back guarantee.

Because all aquatic life are potential carriers of infectious diseases such as atypical mycobacterium and salmonella, always wash your hands before and after handling your aquatic life or habitat contents to help prevent the potential spread of disease.

Pregnant women, children under the age of 5, senior citizens and people with weakened immune systems should contact their physician before purchasing or caring for aquatic life and should consider not having aquatic life as a pet.  

Go to for more information about aquatic life and disease.

The information on this care sheet is not a substitute for veterinary care. If you need additional information, please contact your veterinarian as appropriate.