How to set up a coldwater fish tank

Many of us may have won a goldfish from the fair, or grew up with a goldfish in a bowl. But goldfish and other coldwater fish need a proper aquarium if they are to survive and thrive. Here’s how:

Forget tiny bowls or even nano tanks. If you want to keep goldfish they need a medium to large aquarium from the start. Choose a long, rectangular aquarium which offers swimming space, a large surface area, and room to grow. 2” fish will need a minimum of a 100 litre aquarium, although they will grow quickly and may need a 240 litre aquarium before long. 

Goldfish are messy, producing a lot of solid and liquid waste so they need a powerful filter that can filter mechanically to trap waste and biologically to convert that waste into less harmful substances. The best choice is an external power filter, which has lots of media capacity and can also be packed with additional media like carbon. That will help to remove dyes and odours from the water.

If your budget won’t quite stretch to an external choose an internal canister filter, although the larger the media capacity the better. Despite needing powerful filtration, goldfish don’t appreciate powerful flow, so turn the flow down or deflect it against the glass, plants or ornaments. And if your filter has the capacity to blow bubbles via an inbuilt venturi, the more oxygen, the better. 

Coldwater holds more oxygen than warm water, and coldwater fish like goldfish have a high oxygen demand. That’s all ok if they are living outdoors but the average UK room temperature means that goldfish in aquariums have a high oxygen demand, so an additional airpump is a good idea. You’ll need airline, a non-return valve for safety and an airstone. So those bubbles aren’t just decorative – they’ll help keep your fish healthy and the filter bacteria healthy too. 

The last piece of equipment you’ll “need” is a light. You don’t actually need one at all, but an aquarium light will illuminate the aquarium and show off the colours of the fish. And if you want live plants you’ll definitely need one then too. Choose a fluorescent tube or LED lights which enhances fish colours, and you want to show off those bright reds and oranges in the fish. If you do add plants the light will need to be on for eight hours per day, but if opting for artificial plants or no plants, turn the light off when you’re not watching the fish.

Goldfish need large tanks and efficient filters to cope with their waste

Setting up the tank

Aquariums are heavy so should only be placed on aquarium cabinets which are designed to take the weight. Place the tank away from doorways, windows and radiators, ideally in a place where you can sit and enjoy the fish. 

Wash some smooth, lime-free gravel and place it on the tank base to a depth of about two inches. Add some smooth pebbles, an ornament, and artificial plants, leaving plenty of room for the fish to exercise and swim around the tank.

Fit the filter and airstone, fill with tapwater, and dechlorinate. Switch on the airpump and filter once the tank is full, and add beneficial bacteria either before fish if fishless cycling, or on the day you add the fish, if it says to do so on the bottle. Start with just a few small fish as they will grow, and quickly!

Other useful items

Fancy goldfish should only be fed on sinking goldfish pellets, so you’ll need some of those. But just as essential as the food are test kits to monitor water quality as the tank matures. Test for pH, ammonia, nitrite and nitrate on a regular basis. 

Do you want a background picture on the tank? To maintain the tank you’ll need an algae pad and a gravel vacuum, and don’t forget a clean bucket to syphon water into. Every aquarist needs a net for if they ever need to catch the fish, and it’s always a good idea to have some dechlorinator and filter bacteria spare for water changes and maintenance.   

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Jeremy Gay is an author, lifelong fishkeeper, and aquatic specialist. He's a former editor of Practical Fishkeeping Magazine, UK editor at Reefbuilders, a former aquatic store manager, and has collected fish in Sri Lanka and the Amazon. He's been on tv and radio, contributed to Koi Carp and Gardeners World magazines, been a product tester, a judge, and a product developer. Jeremy is here to guide and advise you on all things tropical, pond and marine, from set-up to stocking, health, feeding to breeding, as well as solving many common fishkeeping problems along the way.


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