Beginner’s Guide To Keeping Pet Fish (Start Here Before You Buy Any)

Fish produce waste, and that waste becomes ammonia. Ammonia burns gills and kills fish at very low concentrations.

In an established tank, ammonia never gets the chance to build up, because colonies of bacteria living in the filter and gravel process it:

1. Fish waste, uneaten food and plant debris produce ammonia, which is highly toxic.
2. One group of bacteria converts ammonia into nitrite, which is also highly toxic.
3. A second group converts nitrite into nitrate, which is much less harmful.
4. You remove the nitrate by changing some of the water every week.

Here is the catch. A brand new tank has none of those bacteria. They take several weeks to establish, and until they do, ammonia and then nitrite rise to lethal levels while the water still looks perfectly clear.

This is called new tank syndrome, and it is the single biggest killer of beginner fish. The water looks clean, the fish start dying, and nothing visible explains it.

The fix is to grow the bacteria first, with no animals in the tank. It is called fishless cycling and it takes two to six weeks.

Set the tank up fully, running filter and heater, then add a source of ammonia, either bottled pure ammonia or a small pinch of fish food daily, and test the water every few days. You will watch ammonia rise and fall, then nitrite rise and fall, then nitrate appear.

The tank is cycled when ammonia and nitrite both read zero and nitrate is present. Only then do fish go in, and only a few at first.

You cannot skip this by waiting. An empty tank with no ammonia source grows no bacteria, so leaving a tank running for a month with nothing in it achieves very little. Bottled bacteria products can speed things up and are worth trying, but test rather than trusting the label.

If you already have fish in an uncycled tank, you are in a fish-in cycle: test daily and do a 25 to 50 percent water change whenever ammonia or nitrite are detectable. It is more work and harder on the fish, but it is recoverable.

You cannot see ammonia, nitrite or nitrate. Without a test kit you are keeping fish blind, and the first symptom you notice will be a dead fish.

Get a liquid test kit that covers ammonia, nitrite, nitrate and pH. They are more accurate than paper strips, which are notoriously unreliable for nitrite in particular, and one kit lasts years.

This is the single most useful thing a beginner can buy, and it is routinely the item people skip.

This is counter-intuitive and it matters. A large volume of water dilutes mistakes and changes temperature and chemistry slowly. A small one swings fast, and a small error becomes fatal quickly.

A 20-gallon tank is a considerably easier first tank than a 5-gallon, and far easier than a bowl. Nano tanks are for people with experience, not beginners.

On bowls, plainly: a goldfish bowl is not suitable for any fish. There is no filtration, no stable temperature, and not enough surface area for gas exchange. It is a decorative object that happens to hold water.

“An inch of fish per gallon.” This rule is wrong and dangerous. It ignores body mass, waste output, territory and swimming needs. A 10-inch common pleco produces vastly more waste than ten 1-inch tetras, and needs a far larger tank than ten gallons. Stock by researching each species, not by arithmetic.

“Fish only grow to the size of their tank.” They do not. A fish kept in too small a tank suffers stunted, deformed growth and organ damage, and dies early. The body stops growing while the internal organs do not. What looks like a conveniently small fish is a damaged one.

Goldfish have a reputation as the easy starter pet, and almost everything about that reputation is wrong.

• They get large. Fancy goldfish reach 6 to 8 inches; common and comet goldfish reach 10 to 14 inches and belong in a pond.
• They live a long time. Ten to twenty years is normal with proper care. The record holder, a goldfish called Tish in England, lived to 43.
• They are messy. Goldfish produce a lot of waste and need strong filtration and large water changes.
• They are cold-water fish. They should not be kept with tropical species, and they do not need a heater.
• They need space. Figures vary, but a single fancy goldfish needs something in the region of 20 to 30 gallons, plus 10 to 20 more for each additional fish.

A goldfish in a bowl typically dies within months. The same fish in a proper tank may outlive the child it was bought for.

Bettas are sold in tiny cups and marketed for vases and desktop ornaments, and the claims attached to them are worth correcting one at a time.

They do not live in puddles. They come from shallow, warm, slow water in Southeast Asia, and the puddle story comes from seeing them in rice paddies, which are large bodies of water.

They do not eat plant roots. Betta vases sold with a plant on top are based on a straightforward falsehood; a betta is a carnivore and needs proper food.

They are tropical and need a heater, a filter and at least five gallons. They breathe air at the surface through a labyrinth organ, which is why they survive dreadful conditions for a while. Surviving is not thriving, and that resilience is precisely what lets them be sold badly.

One male per tank, since two males will fight, and females can be kept together only in larger groups and larger tanks by people who know what they are doing.

These are stocked by general pet shops and sold without warning. The adult size is the problem in most cases.

• Common pleco: sold at 3 inches, grows to 18 or more, needs a very large tank. Also does not keep a tank clean in the way shops imply.
• Iridescent “shark”: a catfish that reaches 3 to 4 feet.
• Bala shark: reaches 14 inches and needs a shoal and a big tank.
• Oscar: intelligent, characterful, reaches 12 to 14 inches and needs 55 gallons or more on its own.
• Common goldfish and comets: pond fish sold as tank fish.
• Chinese algae eater: becomes aggressive with age and stops eating algae.
• Feeder goldfish: kept in dreadful conditions and usually carrying disease.

Genuinely good beginner fish for a cycled 20-gallon tropical tank include platies, guppies, zebra danios, harlequin rasboras, cherry barbs, corydoras catfish in groups of six or more, and a single betta. All are hardy, stay small, and are bred in captivity rather than wild-caught.

Maintenance is straightforward once the tank is established: a 25 percent water change each week, using a gravel vacuum to pull waste out of the substrate, with the replacement water treated with dechlorinator and matched roughly to tank temperature.

Three things people do with good intentions cause real damage:

• Never rinse filter media in tap water. Chlorine kills the bacteria living in it, which is the tank’s entire filtration system. Rinse it gently in a bucket of the water you just removed.
• Never do a full water change or strip and scrub the whole tank. That removes the bacterial colonies and restarts the cycle, which is why a thoroughly cleaned tank often kills its fish a week later.
• Do not replace all the filter media at once. If a cartridge must be changed, run the new one alongside the old for a few weeks first.

Overfeeding is the other common error. Feed once or twice a day, only what is eaten in a couple of minutes, and remove anything left over. Uneaten food becomes ammonia, and a fish can go a week without eating far more comfortably than it can survive poor water.

This is worth knowing before buying, and it splits cleanly.

Most freshwater aquarium fish are captive-bred on farms, chiefly in Southeast Asia, Florida and eastern Europe. Buying them puts no pressure on wild populations.

Most marine aquarium fish are still caught in the wild, and a portion of that catch is taken using sodium cyanide, squirted into reef crevices to stun fish so they can be collected. It damages the reef and kills a high proportion of the fish before they reach a shop. Captive-bred marine fish are increasingly available and worth asking for specifically.

And never release an aquarium fish or plant into a local waterway. Released pets have established damaging populations worldwide, including plecos in Florida and lionfish across the Caribbean. If you cannot keep a fish, rehome it through a local aquarium society or shop.

If you remember five things from this guide, these are the five:

1. Cycle the tank for two to six weeks before buying a single fish.
2. Buy a liquid test kit and actually use it.
3. Get a bigger tank than you think you need; 20 gallons beats 5.
4. Change a quarter of the water weekly, and never wash filter media in tap water.
5. Research the adult size of every fish before you buy it.

Done properly, a freshwater tank is genuinely low-maintenance and will run for years. Nearly every horror story in this hobby comes from the first month, and nearly all of it is avoidable by waiting before adding fish.

And if you want to read about fish rather than keep them, our profiles of the seahorse, the x-ray fish and the eel are a good place to start, the eel in particular, because nobody has ever seen two of them breed.