How Many Fish can you Keep in a Garden Pond

Fish swimming in garden pond

The question of how many fish can be kept in a garden pond comes up a lot, unfortunately it’s a hard for us to give a solid answer too, mainly because there are so many variables to take into account, read on to find out more.

Let us start by saying it’s a great question to ask if you’re asking before you have introduced any fish into a new pond or even if you have an old pond that needs restocking, it shows that you’re going to be a very responsible fish keeper.

A lot depends on the size of the pond you have, the quality of the water and the species and size of fish you would like to keep.

We have heard many stories where people have thought they can add as many fish as they want into their pond thinking everything will be fine, only to find they have so many problems and fish that die.

They then replace those dead fish with new ones and the problems starts all over again.

So let’s try and answer your question and work out how many fish your pond could hold.

How many fish per litre of water.

This is a rough guide to how many fish to a litre of water, but this not the only consideration you should look at when deciding how many fish to have in your pond.

As a rule of thumb it used to be how many fish per litre of water in your pond, but this didn’t take into consideration how big the fish were, so we moved on to how many mm of fish per 50 litres of water in your pond.

Depending on whom you listen to this can be anything from 12.5-25 mm per 50 litres of water for the hobbyist and 50 mm and above per 50 litres of water for the professionals.

To measure your fish you want to take the measurement from the mouth of the fish to the base of the tail fin, you don’t add the tail fin.

So let’s do a simple calculation for 250 mm (25 cm) fish in a pond with 5,000 litres of water.

If you use the 25 mm of fish per 50 litres of water rule which is generally recommended by the industry it would go like this.

  • 250 mm fish ÷ 25 mm = 10. 10 x 50 (litres of water) = 500 litres of water per fish.
  • 5000 litres of water divided by 500 which is the litres of water per fish = 10, so you can have 10 x 250 mm fish in a 5,000 litre pond.

The above is using the 25 mm per 50 litres rule, below is the calculations for 12.5 mm and 50 mm fish per 50 litres.

  • 12.5 mm inch of fish to 50 litres of water works out at 5 x 250 mm fish per 5,000 litres of water.
  • 50 mm of fish to 50 litres of water works out at 20 x 250 mm fish per 5,000 litres of water.

As you can see there is a big difference in the amount of fish you can keep depending on which rule you use.

So why the big difference.

How can you have between 5 and 20 fish in the same sized pond.

Although this is a good starting point to how many fish you could have it doesn’t answer the question of the quality of the water and filtration system your pond has.

You might put the maximum amount of fish that the mm per litre rule suggests, but if the quality of the water is poor your fish will suffer and could end up dying.

Your ponds filter system can determine how many fish your pond can hold .

The filtration of your pond plays a vital role on determining how many fish you can keep in your pond.

The reason you need a filter is to keep the water of your pond clean.

Fish pass waste (poo) and the filter cleans it up along with other waste that needs cleaning.

So let’s use our calculations above and see what type of filter is needed.

If you follow the 12.5 mm of fish per 50 litres of water you will only have 5 x 250 mm fish in your 5,000 litre pond.

With only 5 fish it’s going to be easy to keep your pond clean because there is going to be less fish waste, a simple but effective filtration system is all that’s required.

If you go for the 25 mm of fish per 50 litres of water you will have 10 x 250 mm fish in your 5,000 litre pond.

10 fish are going to produce more waste than 5 fish so you need a good filter system to keep the water clean, but nothing to elaborate.

If you go for 50 mm of fish per 50 litres of water you will have 20 x 250 mm fish producing waste in your 5,000 litre pond, so you have to have an excellent filter system to keep the water clean.

Now you are getting into professional fish keeping .

So a good standard filter should be ample if you don’t overstock your pond but a really good system with multiple filters can sustain a larger population of fish.

It’s always better to have a bigger filter than you actually need though so keep that in mind. 

The more fish you have the more waste they produce, so the better the filtration system you have the better your pond will be cleaned and by doing this your fish will stay healthy.

Test your water: water quality is vital for determining how many fish your pond can handle.

To get an idea of how many fish your pond can take you need to know the quality of your water, so you need to test it.

If you don’t test your water how do you know what’s going on with the water quality, so our advice is to get a water tester kit and test your water.

If the water quality in your pond is toxic your fish are going to suffer and once they are in that water they can’t just get out and look for a new pond to live in that has better conditions, they have to put up with what you give them and over time they might get sick and could die.

Don’t just test the water of new ponds you need to be testing all the time so you act on problems before they get too bad.

For the first couple of months the water of a new pond should be tested weekly and from then on about once a month.

The size of fish is going to be a big factor on how many fish you can have.

When you stock your pond for the first time or when adding new fish you need to take into account the size of fish you buy and how big they will grow.

Its no good calculating that your pond can take 2500 mm of fish so you go and buy 10 young fish at 250 mm long, these fish will grow and Koi can easily reach 600 mm and more so now you have a pond with 6000 mm of fish in in a pond that’s only capable of holding 2500 mm of fish which is somewhat over crowded.

If this happens mother nature can take over and one day you will go to your pond only to find your favourite fish floating on the surface.

If you do find yourself with too many fish in your pond you have a few options, you can sell or give away some of your fish, you can make the pond bigger or you can build a bigger and better filter system.

How many fish you can have depends on the size of your pond.

When working out how many fish you can have in your pond its not just the volume of water you need to take into account, the size of the pond is very important.

Let’s say you have a pond that holds 5,000 litres and using the mm of fish per litre of water rule that means you can have say 10 fish, sounds about right, but what about the surface area.

You see you can have a small but deep 5,000 litre pond with a small surface area or a large but shallow 5,000 litre pond with a large surface area or something in the middle.

But what has surface area got to do with how many fish you can house in your pond, well the surface area governs how much oxygen your pond can can transfer into the water and fish needs oxygen to survive, so a larger surface area is much better than a smaller surface area for providing oxygen to your fish.

And by the way you need to have a pond with at least 2,000 litres of water if you want to keep fish.

Reproduction: this can also have a big effect on the amount of fish in your pond.

Now you might start off with a new pond and a few fish, well within the amount of fish your pond can handle, but happy fish bring little babies.

Before long those babies grow and reproduce, now your pond is starting to get a bit crowded.

As you can see with all the intentions of not overstocking your pond in the first place overcrowding can happen and it doesn’t happen overnight, it sort of creeps up on you.

Again you are faced with getting rid of fish or building a better environment for them all to live in.

So how many fish can you have in your pond.

What we suggest is that if you have a pond with a really good filter, preferably oversized for the size of pond you have and when the water quality is perfect you can start adding fish.

How many fish you add is down to you and how many fish your pond can hold, keep in mind that if too many fish are added to your pond they will get stressed out which lowers their immune system causing them to get sick.

What would be ideal is enough space for your fish to swim gracefully through the water without bumping into each other, if your pond has too many fish which are bumping into each other or can’t swim gracefully it’s time to remove some fish from your pond.

Think of how big your fish will become and using the above advice add fish slowly a couple at a time for about thirty days and keep checking the water quality.

We believe it’s better to have fewer healthy fish than in your pond than to fill to the maximum it can take, it’s much nicer to see a small amount of fish swimming happily around than seeing a lot of fish fighting for space.

Our advice

Once you have calculated how many fish you can have in your pond it’s time to add them, but if you are adding full sized adult fish do not add them all at once .

Instead, test the water, if all is well add a couple of fish, leave it for a couple of weeks and test the water again, if all is still good add some more fish, keep testing the water and adding fish until you have reached the amount of fish your pond will hold or until you’re happy.

Doing it this way will not overwhelm your pond, instead it lets your ponds eco system gradually build up over time.

If at any time when you test the water you find your pond is out of balance do not add more fish, let the pond filter and beneficial bacteria do there job, keep testing and when your pond is back in balance you can continue to add fish.

If on the other hand if you are only going to add baby fish to your pond and let them grow to full size you could probably add them all at once as long as the water and pond are in balance.

Baby fish produce a lot less waste than full sized adult fish, so they are unlikely to upset the balance of your pond.