Fish

Before you set up a tank

• 🦠Cycle the tank before adding fish. Establishing the bacteria that convert ammonia to nitrite and then nitrate takes four to six weeks, and skipping it is what kills most first fish.
• 📦Bigger tanks are easier, not harder. Water parameters swing slowly in a large volume and violently in a small one, which is why a bowl is the hardest possible way to keep a fish.
• 📐Research adult size and temperament before buying. A common pleco reaches forty-five centimetres — buy a bristlenose instead — and a goldfish is a pond animal, not a bowl animal.
• 🧪A test kit is not optional equipment. Ammonia, nitrite, nitrate and pH tell you what is happening before the fish do.
• 🚰Weekly partial water changes of twenty to thirty per cent do more for fish health than any treatment or additive.
• 💧Treat tap water before it goes in. Chlorine and chloramine are added to make it safe for people and they kill the filter bacteria as surely as the fish — which is also why a filter sponge is rinsed in old tank water and never under the tap.
• 🐟Add fish slowly, and quarantine if you can. The filter grows to match the load it has, so a tank stocked in one afternoon poisons itself while the bacteria catch up; and most disease that arrives in an established tank arrives inside a new fish.
• 🤝Check what goes with what before buying, not after. An angelfish will eat a neon tetra, a betta will not tolerate another betta, and a black skirt tetra will spend its day nipping at anything with long fins.
• 🌡️Temperature is part of compatibility, not a dial you set to average. Coldwater fish and tropicals do not belong in the same tank, and a goldfish kept warm lives faster and shorter.

General guidance, not veterinary advice. Anything about a specific animal is a question for a vet who has seen it.

Fish worth knowing about

A place to start rather than the whole list. Every name is a page.

• 🎏
  Betta

  The fish most often kept badly. One male per tank, non-negotiable, but it still wants a heated, filtered, planted tank of reasonable size — not the unheated cup it is sold in.

• 🟠
  Goldfish

  Cold-water, long-lived — fifteen to thirty years, over forty recorded — and far bigger than people expect. Two single-tails want about three hundred litres, or a pond.

• 🧹
  Bronze corydoras

  A small armoured catfish that works the bottom of the tank and is much happier in a group of six or more. Needs soft sand rather than sharp gravel, which damages the barbels.

• 🔺
  Angelfish

  A cichlid, which is the part that catches people out: it pairs up, becomes territorial, and will eat neon tetras sold alongside it. Two random adults forced together often ends badly.

• 🎐
  Koi

  A pond fish, not an aquarium fish, and one that reaches the length of an arm and lives for decades. Filtration and volume are the whole game.

• 🪵
  Bristlenose pleco

  The sensible alternative to the common pleco sold beside it: it stays around twelve centimetres instead of reaching forty-five, and it still wants driftwood to rasp at.

• 

  The four weeks before the fish go in

  Most new fish die of ammonia in a tank that was never cycled. What the nitrogen cycle is, how to run it with no fish in the water, and how to know it worked.

• 

  How to photograph your pet

  Light, height, focus and patience — what separates a good pet photo from a blurred one, and how to handle black cats, white dogs and animals that never sit still.

All 22 fish breeds with photos

Every breed somebody has posted, A to Z.

• Angelfish3
• Anthias3
• Apistogramma2
• Arowana3
• Asagi koi2
• Bala shark2
• Banggai cardinalfish3
• Bekko koi2
• Betta3
• Bichir2
• Black moor3
• Black skirt tetra2
• Blue tang3
• Boesemani rainbowfish3
• Bolivian ram2
• Bristlenose pleco2
• Bronze corydoras2
• Bubble eye goldfish2
• Butterfly koi2
• Butterflyfish3
• Goldfish3
• Koi3