How to Choose the Right Fish for Your Aquarium

There is a particular danger in walking into a good aquarium shop when you have space in one of your tanks.

You know the feeling.

You weren't planning to buy anything. You were only going in for fish food.

Then you see that fish.

You haven't kept one before. It looks fantastic. There are six swimming together in the display tank and you're already mentally rearranging your aquarium to accommodate them.

This is exactly the moment when you need to stop.

Not because buying new fish is a bad idea. That's half the fun of the hobby.

But before taking any fish home, there is one question worth asking:

Can I provide what this fish actually needs?

That question sounds obvious. In practice, it leads to several others.

Here are the seven I think are worth asking before you buy.

1. Does my aquarium actually suit this fish?

Notice that I didn't ask:

“Will it fit in my tank?”

Those are different questions.

A fish can physically fit inside an aquarium without that aquarium providing an appropriate environment for it.

Start with adult size.

The small fish you're looking at in the shop may be a juvenile. Find out how large the species becomes rather than making your decision based on the specimen in front of you.

But size isn't everything.

A small, extremely active fish may need considerably more swimming room than its body length suggests. Some species spend their lives racing backwards and forwards across open water. Others remain close to a small territory.

The shape of the aquarium matters too.

A long aquarium can provide substantially more useful swimming distance than a taller aquarium containing roughly the same volume.

Then consider the environment itself.

Does the species need open swimming space?

Dense planting?

Caves?

Wood?

A sandy bottom?

Strong current?

Very gentle flow?

Plenty of cover?

Your aquarium doesn't merely need enough litres of water.

It needs to provide the right kind of space.

This is why I'm increasingly wary of universal rules such as “one centimetre of fish per litre”. They make stocking sound wonderfully simple while ignoring how differently fish actually live.

Choose the aquarium for the fish, not just the number on the tank.

2. Does this fish need to live with others?

This is one of the most commonly overlooked questions in fishkeeping.

We often choose fish individually.

Fish don't necessarily live that way.

Some species are naturally social and should be maintained with an appropriate number of their own kind. Keeping one individual because you only wanted “one of each” may produce a visually varied aquarium for us, but it may not provide a particularly good social environment for the fish.

This changes the buying decision completely.

Suppose a species grows to only 4 cm.

That sounds tiny.

But if it should be kept as a proper group, the question isn't whether your aquarium has room for one 4 cm fish.

It's whether your aquarium has room for the group.

This is particularly relevant to many tetras, rasboras, danios, Corydoras and other social species.

There is another side to this.

Not every fish wants company.

Some species are territorial. Males may fight with other males. Certain fish form pairs. Others become aggressive when breeding.

“Community fish” is a useful shop label, but it doesn't tell you everything you need to know about social behaviour.

Find out how the fish naturally needs to live before deciding how many to buy.

3. Are my water conditions suitable?

You don't need to become a water chemist before buying your first tetra.

You do, however, need to know something about the water in your aquarium.

Start with temperature.

If your tank normally runs at 24°C and the fish you're considering requires substantially cooler conditions, that's a compatibility problem even if every other fish in the aquarium is peaceful.

Then look at pH and hardness.

Different species have evolved in very different environments. Some come from soft, acidic waters. Others naturally occur in harder, more alkaline conditions.

There can be some flexibility, particularly with captive-bred aquarium strains, but that doesn't mean water parameters should simply be ignored.

My preference for most home aquarists is to turn the question around.

Instead of repeatedly asking:

“How can I change my water to suit this fish?”

ask:

“Which fish are well suited to the water I already have?”

That can make fishkeeping considerably easier.

There are circumstances where experienced aquarists deliberately alter water chemistry, particularly for specialist species and breeding projects.

But for an ordinary community aquarium, constantly fighting your tap water is rarely my first choice.

4. Can I provide the right food in the right way?

Most aquarium fish eat fish food.

That doesn't mean all aquarium fish eat the same food.

Before buying a species, find out what it actually eats.

Is it primarily herbivorous, omnivorous or carnivorous?

Does it readily take flakes or pellets?

Does it benefit from frozen or live foods?

Does it graze throughout the day?

Does it feed at the surface, in the middle of the water column or from the bottom?

That last question matters more than people sometimes realise.

You can put perfectly good food into an aquarium and still fail to feed a particular fish properly.

Imagine an aquarium full of fast, enthusiastic surface and mid-water feeders. Food goes in and there is immediate chaos.

Meanwhile, the shy bottom-dwelling fish you bought last weekend gets whatever happens to survive the journey down.

It isn't enough to ask whether you own suitable food.

Ask whether you can get that food to the fish that needs it.

5. Will it live peacefully with the fish I already have?

This is where the word compatible becomes a little dangerous.

Compatibility isn't simply:

Peaceful + peaceful = compatible.

You need to think about behaviour.

Could one fish eat another?

Could long fins trigger nipping?

Will two species compete aggressively for the same territory?

Is one extremely active while another is easily stressed?

Could a territorial bottom-dweller object strongly to another species moving into exactly the same part of the aquarium?

Don't only research the fish you're buying.

Research the combination.

Think about where each species spends its time as well. A tank may appear to have plenty of room overall while several species are all competing for the same part of it.

And remember that individual behaviour can vary. A species being generally peaceful doesn't guarantee that every individual will behave perfectly.

Aquariums contain animals, not software.

6. Is this particular fish healthy?

You've done your research.

The species is suitable.

Now look at the actual fish in front of you.

Spend a few minutes watching it.

I wouldn't automatically reject a fish simply because it isn't racing around the aquarium. Normal behaviour differs enormously between species.

Instead, ask whether what you're seeing looks normal for that particular species.

Look at its body condition.

Look at the fins.

Look at the eyes and skin.

Watch its breathing.

Is it swimming normally?

Is it maintaining its position properly?

Are there obvious lesions, unusual spots, damaged fins or abnormal swelling?

Is it repeatedly gasping at the surface?

Then look beyond the individual fish.

What do the other fish in the same aquarium look like?

That matters.

A beautiful specimen sharing a tank with several obviously sick fish would make me very cautious.

If possible, also ask to see the fish eat.

A healthy appetite isn't a guarantee of perfect health, but it gives you another useful piece of information.

And what about quarantine?

If you have the ability to quarantine new fish before adding them to an established aquarium, do it.

A separate quarantine tank gives you time to observe new arrivals before exposing every other animal you own to whatever might have arrived with them.

This becomes increasingly important as your collection grows.

The question changes from:

“Can I afford to lose this new fish?”

to:

“What happens if this new fish brings something into the established aquarium?”

Those are very different risks.

7. Am I prepared for the responsibility when life changes?

This is probably the least exciting question on the list.

It might also be one of the most important.

How long does this species live?

What happens when you go away?

What happens if you move house?

What happens during a prolonged power failure?

What if the fish grows considerably larger than expected?

What happens if that innocent pair of livebearers becomes 47 livebearers?

Fishkeeping is usually not difficult on an ordinary Tuesday evening when everything is working.

The real test comes when something changes.

You don't need a disaster-management plan before buying a guppy.

But you should have some idea of the commitment you're making.

Some aquarium fish can live for many years. Larger or specialist species may eventually require equipment, food and aquarium space very different from what they needed when you bought them.

The cheapest part of keeping a fish may well be the price written on the aquarium-shop tank.

Watch: Don't Buy Another Fish Without Asking This First!

I've explored these seven questions in more detail in an episode of The Aquarist's Edge.

If you'd rather hear the discussion, or you'd like to continue exploring the subject after reading this article, you can watch the full episode here:

The video takes a more conversational look at the decisions we make when choosing fish and why that first question matters so much.

So, which fish should I buy?

You may have noticed that I haven't given you a list of the “10 best beginner fish”.

That's deliberate.

A guppy might be an excellent choice in one aquarium and completely unsuitable in another.

The same is true of tetras, Corydoras, rasboras, gouramis, danios and almost every other fish regularly recommended to beginners.

There isn't a universally correct fish.

There is a fish that suits your aquarium, your water, your existing livestock and the care you can realistically provide.

That's a much more useful way to choose.

Take ten minutes before you buy

Most poor fish purchases aren't made because somebody doesn't care about their fish.

They're made because the decision happens too quickly.

You see the fish.

You like the fish.

The price is reasonable.

There appears to be space in the aquarium.

Sold.

Instead, take ten minutes.

Pull out your phone and look up the scientific name. Check its adult size. Find out how it needs to live. Check the temperature and water requirements. Look at what it eats. Research compatibility with the fish you already own.

If something doesn't fit, leave the fish at the shop.

There will be other fish.

And when you do eventually bring something home, you'll know why you chose it.

For me, that is one of the differences between simply owning fish and becoming a fishkeeper.