Fish Husbandry & Aquarium Care

People often think fish are the simplest pet of all — just pop one in a bowl and feed it. In truth, fish are one of the trickiest pets to keep well, because they live in their own toilet. If the water isn't looked after, it slowly poisons them.

Myths, busted

• "A goldfish is happy in a little bowl." No — bowls are far too small, have no filter and no room to swim. A single goldfish needs a large, filtered tank; goldfish grow big and live for many years.
• "Fish only grow to the size of their tank." No — a cramped fish becomes stunted and unwell, not healthy and small.
• "You can add fish the day you set the tank up." No — a brand-new tank isn't safe yet (you'll learn why next).

This is the most important idea in all of fishkeeping. Please read it slowly — once it clicks, everything else makes sense.

Fish make waste, and their food rots. Both release ammonia, which is very poisonous to fish. In a healthy tank, tiny helpful bacteria turn that poison into safer chemicals. The journey looks like this:

• 1Ammonia — from fish waste and old food. Highly toxic, even in small amounts.
• 2Nitrite — one type of good bacteria eats the ammonia and turns it into nitrite. Still toxic.
• 3Nitrate — a second type of bacteria turns nitrite into nitrate. Much less harmful, and you remove it with water changes.

"Cycling" a new tank

A brand-new tank has none of these good bacteria yet. Growing them is called cycling the tank. You run the filter and feed the bacteria (an aquatics shop can advise how) for several weeks before adding any fish, until your tests show ammonia and nitrite have risen and then dropped back to zero.

You can't see ammonia or nitrite — the water can look crystal clear and still be dangerous. The only way to know is to test it. A liquid test kit is more accurate than paper strips.

What to test, and what's safe

• Ammonia — should read 0. Anything above 0 is harmful.
• Nitrite — should read 0. Anything above 0 is harmful.
• Nitrate — kept low. A small amount is normal; water changes keep it down.
• pH — should be stable and suit your species. Sudden pH swings stress fish more than the exact number.

Bigger tanks are actually easier to look after. A large body of water stays more stable — the temperature and water quality change slowly, giving you time to fix problems. Tiny tanks swing quickly and are far less forgiving.

Stocking your tank

• Don't overstock. Too many fish means too much waste for the filter and good bacteria to cope with.
• Research the adult size. That cute little fish in the shop may grow much bigger — plan for its full-grown size.
• Check compatibility. Some species fight, nip fins or eat smaller tank-mates. Some like company; others prefer to be alone.
• Add fish a few at a time so the good bacteria can keep up with the extra waste.

The filter is the heart of the aquarium. It does two big jobs: it moves water around (helping oxygen and flow), and its sponge or media is where most of your good bacteria live. That's why the filter must run 24 hours a day, never switched off overnight.

Looking after your filter

• Choose a filter rated for your tank size (or a little bigger).
• Clean it gently, and only when needed — a filter caked in gunk works poorly.
• Never rinse filter media under the tap. Tap water contains chlorine that kills your good bacteria.
• Instead, rinse the sponge in a bucket of old tank water during a water change. This cleans it while keeping the bacteria alive.

Different fish need different temperatures, and it's vital to match the fish to the tank.

• Tropical fish (like tetras, guppies and bettas) need a warm tank, kept steady with a heater.
• Coldwater fish (like goldfish) don't need a heater, but still need a stable, cool temperature and plenty of space.
• Use a thermometer so you can check the temperature every day.

It's easy to love your fish by overfeeding them — but that does real harm. Uneaten food rots, releasing ammonia and polluting the water.

Feeding well

• Feed little and often — only as much as the fish eat in a couple of minutes.
• Feed the right food for your species (some need flakes, some pellets, some plants or sinking foods).
• Remove any uneaten food so it doesn't rot in the tank.
• A fish missing the odd meal is fine — a polluted tank is far more dangerous than a slightly hungry fish.

Even with a good filter, nitrate slowly builds up. The way we keep it down is the partial water change — swapping out some of the old water for fresh. This is the single most important routine job in fishkeeping.

Doing a water change

1. Change a small part of the water regularly (a modest amount, often), rather than a huge amount all at once.
2. Gravel vacuum to lift out waste and old food from the bottom.
3. Treat the new tap water first — tap water contains chlorine and chloramine, which harm fish and kill your good bacteria. Always add a dechlorinator (water conditioner) to fresh water before it goes in.
4. Match the temperature of the new water to the tank so the fish aren't shocked.

A new fish arrives in a bag of water that may be a different temperature and chemistry from your tank. Dropping it straight in is a shock. We ease it in gently — this is called acclimatising.

1. Float the sealed bag in the tank for a while so the temperatures slowly match.
2. Mix a little of your tank water into the bag, bit by bit, so the new fish adjusts to your water.
3. Gently net the fish into the tank — try not to tip lots of shop water in with it.
4. Keep the lights low and leave the fish in peace to settle.

Learning to spot a poorly fish early gives it the best chance. Watch your fish each day — you'll quickly learn what "normal" looks like for them.

Common warning signs

• White spot (ich): tiny white dots, like grains of salt, over the fins and body.
• Fin rot: ragged, frayed or shrinking fins.
• Gasping at the surface: often a sign of poor water quality or low oxygen.
• Clamped fins: fins held tight against the body instead of spread out.
• Loss of appetite, hiding away, unusual swimming or rubbing against objects.

Tick off each item as you prepare your tank. Your progress saves automatically. Remember: set it all up and cycle the tank before buying fish.

• A tank as large as you can fit and afford
• A filter rated for the tank size
• Heater (for tropical fish) & a thermometer
• Liquid water test kit (ammonia, nitrite, nitrate, pH)
• Dechlorinator / water conditioner
• Gravel or substrate, rinsed clean
• Plants, hides & decorations for shelter
• Gravel vacuum & a clean bucket (fish-only)
• A well-fitting lid or hood
• The right food for your chosen species
• A fish net
• Time to cycle the tank before buying any fish

Decide what you would do. Tap your answer, then read the guidance.