This is one of the most frequently asked questions, and one that we’ll attempt to answer here. But it is subject to many variables which need to be addressed first. 

Fish need food, water, oxygen and ammonia-free water in order to stay alive. Fish consume oxygen, lowering it, and release CO2 as they breathe, raising it. Low oxygen can kill fish as can high CO2 levels, so the surface area of the aquarium must be sufficiently large to facilitate Oxygen diffusing into the water and CO2 leaving it. 


Without any water movement from a pump or airstone, how many fish you keep in an aquarium is based on that one factor in the short term. But agitate the surface of the water with the filter outlet or better still an airstone, powered by an airpump, and more oxygen is brought into the aquarium, while more CO2 is gassed off.


But an aquarium with sufficient oxygen is only good if the water is free of toxic substances such as ammonia. Fish release ammonia from their gills as they breathe, yet they are not adapted to live in water that contains it. This is because, in nature, the size of the water body, be it a river or lake, dilutes the ammonia, as well as there is a plethora of bacteria, plants and microorganisms present to consume or convert it.

So place a fish into an unaerated aquarium and it may succumb to low oxygen first. But place it into an oxygenated but unfiltered aquarium, and ammonia will surely cause its untimely death. 

Use a filter with biological media however, and ammonia can be converted by beneficial bacteria first into nitrite (also very toxic,) then into nitrate, which is far less so. It’s biological filters that are the real enablers when it comes to us keeping any fish alive in aquariums. 

How many fish?

So then we are back to the original question of how many fish you can keep in an aquarium or pond. The answer is that you can keep as many as the biological filter and aeration can cope with. In simple terms, it’s not about how many fish, like 10 neons, five platies, three corydoras etc, but actually how much ammonia they cumulatively produce and how much oxygen they consume.

More ammonia than the biological filter can cope with and the fish will all die. More oxygen consumption than is going into the water, and they will asphyxiate. 

Fish farming

In fish farming, they need to know the answer to this question too. They want to keep the maximum number of fish they can in a body of water in order to gain maximum yield, and they want to feed them as much as they possibly can to grow them up quickly, without risking poisoning them with their own pollution.

There they use equations involving kilograms of cumulative fish weight per 1000 litres of water, and different species have maximum stocking limitations before there’s a problem, as they all have different oxygen requirements and pollution tolerances. 

For ornamental fish, experts at OATA have put together some similar guidelines on the maximum stocking recommendations for aquatic wholesalers and retailers to use as a guide, but it’s still difficult to equate that to actual numbers of Neon tetras per tank without taking the fish out and weighing them, which we don’t recommend you do. 

But if you assume that an adult goldfish weighs about half a kg with an 8kg/1000 litres recommended maximum, 16 fully grown goldfish in a 1000 litre pond sounds about right. With as little as 1 and 1.5kg being recommended per 1000 litres of aquarium water for tropical and marine fish. 

Cold Water Species

8kg/1000 litres

Tropical Freshwater Species

Fish up to 5cm (or 2”) - 1.5kg/1000 litres

Fish over 5cm (or 2”) - 2.5kg/1000 litres

Tropical Marine Species

Fish up to 5cm (or 2”) - 1kg/1000 litres

Fish over 5cm (or 2”) - 2kg/1000 litres

So it’s not as vague as to how long is a piece of string, and there is lots of science behind it, at least in fish farming. But if you massively increase biological filtration capacity, aeration and water changes, you can keep more fish in a given tank volume than if you have a small filter with no extra aeration. 

Here at Swell UK we recommend you take stocking advice from expert aquatic staff, books and magazines, and experienced aquarists. Always use a test kit to monitor ammonia, nitrite and nitrate and if you register even a low level of ammonia in an established tank, it may mean that you have more fish and are feeding them more food, than your filter can cope with.