Fish care basics: start with the water

Cycle the tank before fish arrive

Fish waste produces ammonia, which is toxic. In a healthy tank, beneficial bacteria in the filter turn ammonia into nitrite and then into nitrate, which is much less harmful and is removed by water changes. Building up those bacteria is called cycling, and it often takes several weeks. Set up the tank, add a source of ammonia, and test the water with a liquid test kit until ammonia and nitrite both read zero. Ask your local aquarium store about fish-free cycling methods.

Tank size

Bigger tanks are more stable and easier to keep healthy. Goldfish grow large and produce a lot of waste. They need a big filtered tank or a pond. A bowl is too small to keep clean. Many keepers start at around 20 gallons for one fancy goldfish and add more for each extra fish; common goldfish and comets are better suited to ponds. Bettas need a heated, filtered tank of at least 5 gallons, and more room is welcome.

Temperature

Goldfish are coldwater fish and do well at room temperature in most homes. Bettas are tropical and need a heater, commonly set somewhere around 76 to 82°F (24 to 28°C). Use a thermometer and avoid putting tanks in direct sun or near heating vents, which cause temperature swings.

Water changes and testing

• Change part of the water every week. Many keepers replace around a quarter each time.
• Treat tap water with a water conditioner to remove chlorine and chloramine.
• Match the new water's temperature to the tank.
• Rinse filter media in old tank water, never under the tap, so the bacteria survive.
• Test ammonia, nitrite and nitrate regularly, and any time the fish act oddly.

Feeding

Overfeeding is one of the most common mistakes. Feed small amounts that the fish finish within a couple of minutes, once or twice a day, and remove leftovers. Use a food made for the species: bettas are carnivores and need a protein-rich pellet, and goldfish do well on sinking pellets plus some blanched vegetables.

Adding new fish

Add fish slowly so the filter bacteria can keep up. Many keepers hold new fish in a separate quarantine tank for a few weeks to watch for disease. Research adult size and temperament before buying; male bettas will fight each other, and fin-nipping species can harass them.

Never release pet fish or aquarium plants into lakes, rivers or storm drains. Released goldfish can survive and harm native wildlife, and releasing them is illegal in many places.

Sick fish

Test the water first, since most fish illness starts with water quality. Some vets treat fish, and an aquatic vet can advise before you add medicines to a tank.

Fish care: questions