Article Hero Image

FISH / health & care

Healthy Aquarium Water

Fish need clean, chemically balanced water to thrive.

Overview

Your aquarium’s environment

An aquarium is a delicately balanced ecosystem that relies on “good” bacteria to keep fish healthy. When everything is working right, helpful bacteria grow in the aquarium’s gravel and filter. There, they help break down the ammonia and nitrite found in fish waste—which, if left unchecked, would be toxic to your fish.

When you first set up your aquarium, you’ll need to encourage the growth of these beneficial bacteria. The process, called cycling, can take 4 to 6 weeks. Using bacteria starters can reduce the cycling time to 1 to 3 weeks. Here’s how to start:

  1. Set up and fill the aquarium with water. Do not add fish. Run the aquarium filter for at least 24 hours.
  2. Introduce a small number of danios, gouramis or livebearers—hardy fish that aren’t as sensitive to ammonia or nitrites. (The number of fish will depend on the size of the aquarium; ask a PetSmart associate for advice.)
  3. Use a cycling-aid product to seed the aquarium with healthy bacteria.
  4. Feed your starter fish carefully, according to package directions.
  5. After a few days, use a testing kit to determine the levels of ammonia, nitrites and pH in the aquarium water (PetSmart offers free water testing. Just bring in a sample and we’ll test it for you in minutes.) If levels of any of these chemicals are out of balance, remove some of the water from the aquarium and replace it with clean water.
  6. Over time, the beneficial bacteria should kick in and balance the chemistry in the aquarium. Once the water is ready, you can slowly introduce new fish.

Aquarium water care schedule

Daily

  • Be sure to check the water temperature in your tank every day.

Weekly

  • replace 10% of the tank with clean water
  • test the pH, ammonia, nitrite, nitrate, hardness, alkalinity and chlorine
  • clean away any algae

Monthly

  • vacuum aquarium gravel
  • replace 25% of the tank with clean water
  • inspect filter; rinse or replace the cartridge, carbon and pre-filter if necessary
  • clean away any algae
  • remove and clean plastic plants and decorative items
  • replace air stone
  • prune live plants if necessary

Water test log

pH levels

  • Normal pH range: 6.5 to 8.2
  • Rapid changes in pH levels are unhealthy for your fish

chlorine and chloramine

  • Normal results: 0.0 mg/L
  • The chlorine in tap water is toxic to fish. Dechlorinate any water before adding it to the aquarium.

ammonia

  • Normal results: 0.0 to 0.25 mg/L
  • The water in a new aquarium may have a small amount of ammonia, which should disappear completely after the beneficial bacteria are growing. If it rises again, change out some water to lower the ammonia to a safe level.

nitrite

  • Normal results: 0.0 to 0.5 mg/L
  • Nitrite reduces the oxygen in your fish’s blood. If nitrite levels spike, change the water until it is at a safe level. You can also add aquarium salt to your tank (use 1 to 3 teaspoons per gallon).

nitrate

  • Normal range: 0 to 40 mg/L
  • Nitrate encourages algae, so keep the levels low. A water change will help lower nitrate levels.

hardness

  • Normal range: 100 to 250 mg/L
  • This is the calcium and magnesium level in the water. Most fish are ok at different hardness levels. Test you tap water hardness and make water changes as needed.

alkalinity

  • Normal range: 120 to 300 mg/L
  • Low alkalinity can result in a sudden change in the tank’s pH level. Test tap water alkalinity and change water as needed to maintain proper alkalinity.

temperature

  • Normal range: 74 to 82 degrees Fahrenheit. (23 to 28 C)
  • Sudden swings in temperature can harm fish. Use an aquarium heater to keep a consistent temperature.
Shop water test kits Shop water conditioners

Filter facts

There are three different kinds of aquarium filtration: mechanical, biological and chemical. The most important of these is biological filtration. However, your aquarium needs all three to neutralize ammonia and nitrites, and remove floating debris and contamination. These common filtration systems employ all three methods:

  • Under-gravel filter: A slotted plate under the gravel bed circulates water continuously.
  • Power filter: The favorite for tanks up to 55 gallons, a power filter can be used along with an under-gravel filter. The combination can be helpful if you’re keeping a big school in your aquarium.
  • External power filter: Located on the back of the aquarium, this style draws water from the tank and pumps it through a replaceable filter cartridge.
  • Canister filter: Ideal for aquariums 55 gallons and bigger.
Shop filters

Ready to find your fish?

We have a wide selection of freshwater fish sold in stores.